Our ability to commercialize pharmaceutical products with collaborators may depend, in part, on the extent to which reimbursement for the products will be available from:

•

government and health administration authorities;



•

private health insurers; and



•

other third-party payors.

        Significant uncertainty exists as to the reimbursement status of newly-approved healthcare products. Third-party payors, including Medicare, are challenging the prices charged for medical products and services. Government and other third-party payors increasingly are attempting to contain healthcare costs by limiting both coverage and the level of reimbursement for new drugs and by refusing, in some cases, to provide coverage for uses of approved products for disease indications for which the FDA has not granted labeling approval. Third-party insurance coverage may not be available to patients for any products we discover and develop, alone or with collaborators. If government and other third-party payors do not provide adequate coverage and reimbursement levels for our products, the market acceptance of these products may be reduced.

If product liability lawsuits are successfully brought against us, we may incur substantial liabilities and may be required to limit commercialization of our products.

        The testing and marketing of medical products entail an inherent risk of product liability. If we cannot successfully defend ourselves against product liability claims, we may incur substantial liabilities or be required to limit commercialization of our products. We currently do not have product liability insurance, and our inability to obtain sufficient product liability insurance at an acceptable cost to protect against potential product liability claims could prevent or inhibit the commercialization of pharmaceutical products we develop, alone or with corporate collaborators. We, or our corporate collaborators, might not be able to obtain insurance at a reasonable cost, if at all. While under various circumstances we are entitled to be indemnified against losses by our corporate collaborators, indemnification may not be available or adequate should any claim arise.

Our research and development efforts will be seriously jeopardized, if we are unable to attract and retain key employees and relationships.

        As a small company with only 134 employees as of May 15, 2003, our success depends on the continued contributions of our principal management and scientific personnel and on our ability to develop and maintain important relationships with leading academic institutions, scientists and companies in the face of intense competition for such personnel. In particular, our research programs depend on our ability to attract and retain highly skilled chemists, other scientists, and regulatory and clinical personnel. If we lose the services of any of our personnel, our research and development efforts could be seriously and adversely affected. Our employees can terminate their employment with us at any time.

We depend on various scientific consultants and advisors for the success and continuation of our research efforts.

        We work extensively with various scientific consultants and advisors. The potential success of our drug discovery programs depends, in part, on continued collaborations with certain of these consultants and advisors. We, and various members of our management and research staff, rely on certain of these consultants and advisors for expertise in our research, regulatory and clinical efforts. Our scientific advisors are not employees of ours and may have commitments to, or consulting or advisory contracts with, other entities that may limit their availability to us. We do not know if we will be able to maintain such consulting agreements or that such scientific advisors will not enter into consulting arrangements,

15

exclusive or otherwise, with competing pharmaceutical or biotechnology companies, any of which would have a detrimental impact on our research objectives and could have a material adverse effect on our business, financial condition and results of operations.

If we use biological and hazardous materials in a manner that causes injury or violates laws, we may be liable for damages.

        Our research and development activities involve the controlled use of potentially harmful biological materials as well as hazardous materials, chemicals and various radioactive compounds. We cannot completely eliminate the risk of accidental contamination or injury from the use, storage, handling or disposal of these materials. In the event of contamination or injury, we could be held liable for damages that result, and such liability could exceed our resources. We are subject to federal, state and local laws and regulations governing the use, storage, handling and disposal of these materials and specified waste products. The cost of compliance with, or any potential violation of, these laws and regulations could be significant.

Our facilities are located near known earthquake fault zones, and the occurrence of an earthquake or other catastrophic disaster could cause damage to our facilities and equipment, which could require us to cease or curtail operations.

        Our facilities are located in the San Francisco Bay Area near known earthquake fault zones and are vulnerable to significant damage from earthquakes. We are also vulnerable to damage from other types of disasters, including fires, floods, power loss, communications failures and similar events. If any disaster were to occur, our ability to operate our business at our facilities would be seriously, or potentially completely, impaired, and our research could be lost or destroyed. In addition, the unique nature of our research activities and of much of our equipment could make it difficult for us to recover from a disaster. The insurance we maintain may not be adequate to cover or losses resulting from disasters or other business interruptions.

If our officers, directors and largest stockholders choose to act together, they may be able to significantly affect our management and operations, acting in their best interests and not necessarily those of other stockholders.

        Our directors, executive officers and principal stockholders and their affiliates beneficially own approximately 46% of our common stock, based on their beneficial ownership as of April 29, 2003. Accordingly, they collectively will have the ability to significantly affect the election of all of our directors and the outcome of most corporate actions requiring stockholder approval. They may exercise this ability in a manner that advances their best interests and not necessarily those of other stockholders.

        On April 29, 2003, we entered into a definitive agreement for the sale of $46.0 million of newly issued shares of common stock and warrants to purchase common stock in a private placement led by MPM Capital, L.P. that includes Frazier Healthcare, Alta Partners and HBM BioVentures. Under the terms of the agreement, we have agreed to issue to the investors 71,874,999 shares of common stock at a price of $0.64 per share and warrants to purchase an additional 14,374,997 shares of common stock at an exercise price of $0.64 per share. As a result of their combined approximate 70% ownership (without giving effect to the exercise of the warrants and assuming 46,376,0004 shares outstanding on April 29, 2003), the investors would have control over Rigel following the closing of the transactions contemplated by the purchase agreement. If we successfully consummate the proposed private placement, the investors would hold the requisite percentage of our outstanding shares so as to permit them, if they chose to act in concert, to take actions requiring stockholder approval without obtaining the approval of our other stockholders. In addition, we would use our commercially reasonable best

16

efforts to elect designees of MPM Capital as two of our nine board members as of the closing of the private placement.

Our stock price may be volatile, and your investment in our stock could decline in value.

        The market prices for our securities and those of other biotechnology companies have been highly volatile and may continue to be highly volatile in the future. The following factors, in addition to other risk factors described in this section, may have a significant impact on the market price of our common stock:

•

the receipt or failure to receive the significant amount of additional funding necessary to conduct our business;



•

the progress and success of preclinical studies and clinical trials of our drug candidates conducted by us or our collaborative partners or licensees;



•

announcements of technological innovations or new commercial products by our competitors or us;



•

developments concerning proprietary rights, including patents;



•

developments concerning our collaborations;



•

publicity regarding actual or potential medical results relating to products under development by our competitors or us;



•

regulatory developments in the United States and foreign countries;



•

litigation;



•

economic and other external factors or other disaster or crisis; and



•

period-to-period fluctuations in financial results.

Anti-takeover provisions in our charter documents and under Delaware law may make an acquisition of us, which may be beneficial to our stockholders, more difficult.

        Provisions of our amended and restated certificate of incorporation and bylaws, as well as provisions of Delaware law, could make it more difficult for a third party to acquire us, even if doing so would benefit our stockholders. These provisions:

•

establish that members of the board of directors may be removed only for cause upon the affirmative vote of stockholders owning at least two-thirds of our capital stock;



•

authorize the issuance of "blank check" preferred stock that could be issued by our board of directors to increase the number of outstanding shares and thwart a takeover attempt;



•

limit who may call a special meeting of stockholders;



•

prohibit stockholder action by written consent, thereby requiring all stockholder actions to be taken at a meeting of our stockholders;



•

establish advance notice requirements for nominations for election to the board of directors or for proposing matters that can be acted upon at stockholder meetings; and



•

provide for a board of directors with staggered terms.

        In addition, Section 203 of the Delaware General Corporation Law, which imposes certain restrictions relating to transactions with major stockholders, may discourage, delay or prevent a third party from acquiring us.

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FORWARD LOOKING STATEMENTS

        Some of the statements in this prospectus and the documents incorporated by reference other than statements of historical fact are forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and within the meaning of Section 21E of the Securities Exchange Act of 1934, as amended, that are subject to the "safe harbor" created by those sections. These forward-looking statements include, but are not limited to, statements about:

•

risks associated withregarding the successfollowing: our business and scientific strategies; the progress of our product development programs, including clinical testing; our corporate collaborations, including revenues received from these collaborations; our drug discovery technologies; our research and product development programs;



•

results achieved in future preclinical studies and clinical trials;



•

expenses; protection of our intellectual property; sufficiency of our cash resources;



•

dependence on revenues from existing and new collaborations;



•

uncertainty of product development, need for additional capital and uncertainty of change;



•

our research and development and other expenses;



•

our operations and legal risks;



•

governmental regulation and the regulatory approval process;



•

uncertainty of health care reform measures;



•

uncertainty of potential proprietary rights;



•

the scope and validity of patents;



•

our proprietary technology and corporate partnerships;



•

dependencerisks. You should not place undue reliance on key personnel;



•

history of operating losses and anticipation of future losses;



•

competitive technologies and products;



•

management of growth and risks of acquiring new technologies; and



•

our ability to complete the rights offering and the timing thereof.

        These forward-looking statements are generally identified by words such as "expect," "anticipate," "intend," "believe," "hope," "assume," "estimate," "plan," "will" and other similar words and expressions. Discussions containing these forward-looking statements, may be found, among other places,which apply only as of the date of this prospectus. Our actual results could differ materially from those anticipated in "Business" and "Management's Discussion and Analysis of Financial Condition and Results of Operations" incorporated by reference from our most recent annual report on Form 10-K, as well as any amendments thereto reflected in subsequent filings with the Securities and Exchange Commission. Thesethese forward-looking statements involve risksfor many reasons. Any forward-looking statement speaks only as of the date on which it is made, and uncertainties that couldwe undertake no obligation to update any forward-looking statement to reflect events or circumstances after the date on which the statement is made or to reflect the occurrence of unanticipated events. New factors emerge from time to time, and it is not possible for us to predict which factors will arise. In addition, we cannot assess the impact of each factor on our business or the extent to which any factor, or combination of factors, may cause our actual results to differ materially from those contained in theany forward-looking statements. Reference is made to discussion about risks that may affect our business under "Risk Factors" above. We do not undertake any obligation to update forward-looking statements. The risks contained in this prospectus, among other things, should be considered in evaluating our prospects and future financial performance.

USERATIO OF PROCEEDS
EARNINGS TO FIXED CHARGES

        Our gross proceeds from the rights offering depend on the number of shares that are purchased. If allearnings were insufficient to cover fixed charges in each of the subscription rights offered by thisyears in the five-year period ended December 31, 2002 and in the nine-month period ended September 30, 2003. "Earnings" consist of income (loss) from continuing operations before income taxes, extraordinary items, cumulative effect of accounting changes, equity in net losses of affiliates and fixed charges. "Fixed charges" consist of interest expense and the portion of operating lease expense that represents interest. The extent to which earnings were insufficient to cover fixed charges is as follows:

USE OF PROCEEDS

        Except as described in any prospectus are exercised, thensupplement, we will receive approximately $10 million. We currently intend to use the net proceeds from the rights offeringsale of our securities for research and development and general corporate purposes. We may also use a portion of the net proceeds to acquire or invest in businesses, products and technologies that are complementary to our own, although we

18

currently are not planning or negotiating any such transactions. Pending these uses, the net proceeds will be invested in investment-grade, interest-bearing securities.

THE RIGHTS OFFERING
DESCRIPTION OF CAPITAL STOCK

        Before exercising any subscription rights, you should read carefully the information set forth under "Risk Factors."

The subscription rights

        We will distribute to each holderOur authorized capital stock consists of our100 million shares of common stock, on the record date, which was 5:00 p.m. Central Daylight time on April 29, 2003, at no charge, one non-transferable subscription right for each share of our common stock owned. If all subscription rights are exercised, we will sell a total of approximately 15,625,000$0.001 par value, and 10 million shares of our common stock. The subscription rights will be evidenced by non-transferable subscription rights certificates. Although we will distribute subscription rights to eachpreferred stock, $0.001 par value. As of our stockholders on the record date, the private placement investors have waived any subscription privileges in order to enhance the subscription privileges of our other common stockholders.

        Each subscription right will allow you to purchase    of a share of our common stock, rounded down to the nearest whole number, at the subscription price of $0.64 per share. If you elect to exercise your basic subscription privilege in full, you will also be entitled to subscribe, at the subscription price, for additionalDecember 31, 2003, there were 14,828,546 shares of our common stock under your over-subscription privilegeoutstanding and no shares of preferred stock outstanding. In addition, certain stockholders held warrants to purchase 1,724,953 shares of our common stock.

4

Common Stock

        The holders of common stock are entitled to one vote for each share held of record on all matters submitted to a vote of the extent that other stockholders and do not exercisehave cumulative voting rights. Accordingly, holders of a majority of the shares of common stock entitled to vote in any election of directors may elect all of the directors standing for election. Subject to preferences that may be applicable to any outstanding shares of preferred stock, the holders of common stock are entitled to receive ratably such dividends as may be declared by the board of directors out of funds legally available therefor. Upon the liquidation, dissolution or winding up of Rigel, holders of our common stock are entitled to share ratably in all assets remaining after payment of liabilities and the liquidation preferences of any outstanding shares of preferred stock. Holders of common stock have no preemptive rights and no right to convert their basic subscription privilegescommon stock into any other securities. There are no redemption or sinking fund provisions applicable to our common stock. All outstanding shares of common stock are, and all shares of common stock that may be issued under this prospectus will be, fully paid and non-assessable.

Preferred Stock

        Pursuant to our amended and restated certificate of incorporation, our board of directors has the authority, without further action by the stockholders, to issue up to 10 million shares of preferred stock, in full. Ifone or more series. Our board of directors is authorized to fix or alter from time to time the designation, powers, preferences and rights of the shares of each series, including dividend rights, conversion rights, voting rights, terms of redemption, liquidation preferences and sinking fund terms, as well as the qualifications, limitations or restrictions of any unissued series of preferred stock. Our board of directors may also establish from time to time the number of shares available after satisfactionconstituting any series of all basic subscriptions is insufficientpreferred stock, and to satisfy fully all elections to exercise the over-subscription privilege, we will allocate the excess shares pro rata among those over-subscribing. We will base the pro rata allocation onincrease or decrease the number of shares subscribed for pursuantof any series subsequent to the basic subscription privilege. The opportunity to exerciseissuance of shares of that series, but not below the over-subscription privilege is available to all subscription rights holders on the same terms.

        If you hold yournumber of shares in a brokerage account or by a custodian bank or other nominee, you will not receive a subscription rights certificate, and your subscription rights must be exercised through the broker, custodian bank or other nominee. The following describes the rights offering in general and assumes (unless specifically provided otherwise) that you are a record holder of our common stock. If you hold your shares in a brokerage account or by a custodian bank or other nominee, please contact your broker, custodian bank or other nominee to participate in the rights offering.

        If you hold your shares directly, you will receive a non-transferable subscription rights certificate. As a holder of subscription rights you will be entitled to two subscription privileges: (1) a basic subscription privilege and (2) an over-subscription privilege. These privileges are described below.any series then outstanding.

        We will not issue fractional sharesfix the rights, preferences, privileges and restrictions of the preferred stock of each series in the rights offering, but rathercertificate of designation relating to that series. We will round down any fractional sharesincorporate by reference as an exhibit to the nearest whole share. For example, if you exercise 100 subscription rights, you will receive    shares of our common stock, instead of            shares of our common stock you would have received without rounding.

        Your purchase of shares of our common stock pursuantregistration statement that includes this prospectus or as an exhibit to a current report on Form 8-K, the rights offering is not conditioned upon the subscriptionform of any minimum numbercertificate of shares by you and the other holders of the subscription rights.

Before exercising any subscription rights, you should read the information set forth under "Risk Factors" beginning on page 5 carefully.

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Expiration date; amendments and termination

        You may exercise the basic subscription privilege and the over-subscription privilege at any time before 5:00 p.m. Central Daylight time on                         , 2003, the expiration date for the rights offering. We may, in our sole discretion, extend the time for exercising the subscription rights. We will not extend the date the subscription rights expire beyond            , 2003, unless our board of directors believesdesignation that a material event has occurred and we need more time to disclose adequately to you the information about the event. If the commencement of the rights offering is delayed for a period of time, the expiration date of the rights offering will be similarly extended. If we elect to extend the date the subscription rights expire, we will issue a press release announcing the extension before the first Nasdaq National Market trading day after the most recently announced expiration date.

        We reserve the right, in our sole discretion, to amend, terminate or modifydescribes the terms of the rights offering. Ifseries of preferred stock we terminate the rightsare offering all affected subscription rights will expire without value and we will promptly return all of your subscription payments to you, without interest or deduction.

        If you do not exercise your subscription rights before the time they expire, then your subscription rights will be null and void. We will not be obligated to honor your exercise of subscription rights if the subscription agent receives the documents relating to your exercise after the time they expire, regardless of when you transmitted the documents, except when you have timely transmitted the documents pursuant to the guaranteed delivery procedures described below.

Subscription privileges

        Your subscription rights entitle you to the basic subscription privilege and the over-subscription privilege.

        Basic Subscription Privilege.    With the basic subscription privilege, you may purchase            of a share of our common stock, rounded down in the aggregate to the nearest whole number, per subscription right, upon deliveryissuance of the required documents and paymentrelated series of the subscription price of $0.64 per share, before the time the subscription rights expire. You are not required to exercise all of your subscription rights unless you wish to purchase shares under your over-subscription privilege. Wepreferred stock. This description will deliver to the record holders who purchase share in the rights offering certificates representing the shares purchased with a holder's basic subscription privilege as soon as practicable after the rights offering has expired.

        Over-Subscription Privilege.    In addition to your basic subscription privilege, you may subscribe for additional shares of our common stock, upon delivery of the required documents and payment of the subscription price of $0.64 per share before the time the subscription rights expire, if you exercised your basic subscription privilege in full and other holders of subscription rights do not exercise their basic subscription privileges in full.

        Pro Rata Allocation.    If there are not enough shares to satisfy all subscriptions pursuant to the exercise of the over-subscription privilege, we will allocate the remaining shares pro rata (subject to the elimination of fractional shares) among those over-subscribing. Pro rata means in proportion to the number of shares you and the other holders have purchased by exercising the basic subscription privileges. If there is a pro rata allocation of the remaining shares and the pro ration results in the allocation to you of a greater number of shares than you subscribed for pursuant to the over-subscription privilege, then we will allocate to you only the number of shares for which you subscribed. We will allocate the remaining shares among all other holders exercising their over-subscription privilege.

        Full Exercise of the Basic Subscription Privilege.    You may exercise the over-subscription privilege only if you exercise your basic subscription privilege in full. To determine if you have fully exercised

20

your basic subscription privilege, we will consider only the basic subscription privileges held by you in the same capacity. For example, suppose you were granted subscription rights for shares of our common stock you own individually and shares of our common stock you own collectively with your spouse. If you wish to exercise your over-subscription privilege with respect to the subscription rights you own individually, but not with respect to subscription rights you own collectively with your spouse, you only need to exercise your basic subscription privilege with respect to your individually owned subscription rights. You do not have to subscribe for any shares under the basic subscription privilege owned collectively with your spouse to exercise your individual over-subscription privilege.

        When you complete the portion of the subscription rights certificate to exercise your over-subscription privilege, you will be representing and certifying that you have fully exercised your basic subscription privilege as to shares of our common stock you hold in that capacity. You must exercise your over-subscription privilege at the same time you exercise your basic subscription privilege in full.

        If you own your shares of our common stock through your bank, broker or other nominee holder who will exercise your over-subscription privilege on your behalf, the nominee holder will be required to certify to us and the subscription agent:include:

•

the number of shares held on the record date on your behalf;



•

the number of subscription rights you exercised under your basic subscription privilege;



•

that your entire basic subscription privilege held in the same capacity has been exercised in full;title and stated value;



•

the number of shares we are offering;



•

the liquidation preference per share;



•

the purchase price;



•

the dividend rate, period and payment date and method of calculation for dividends;



•

whether dividends will be cumulative or non-cumulative and, if cumulative, the date from which dividends will accumulate;



•

the procedures for any auction and remarketing, if any;



•

the provisions for a sinking fund, if any;



•

the provisions for redemption or repurchase, if applicable, and any restrictions on our ability to exercise those redemption and repurchase rights;



•

any listing of the preferred stock on any securities exchange or market;



•

whether the preferred stock will be convertible into our common stock, you subscribed for pursuantand, if applicable, the conversion price, or how it will be calculated, and the conversion period;

5

•

whether the preferred stock will be exchangeable into debt securities, and, if applicable, the exchange price, or how it will be calculated, and the exchange period;



•

voting rights, if any, of the preferred stock;



•

preemption rights, if any;



•

restrictions on transfer, sale or other assignment, if any;



•

whether interests in the preferred stock will be represented by depositary shares;



•

a discussion of any material or special United States federal income tax considerations applicable to the over-subscription privilege.preferred stock;



•

the relative ranking and preferences of the preferred stock as to dividend rights and rights if we liquidate, dissolve or wind up our affairs;



•

any limitations on issuance of any class or series of preferred stock ranking senior to or on a parity with the series of preferred stock as to dividend rights and rights if we liquidate, dissolve or wind up our affairs; and



•

any other specific terms, preferences, rights or limitations of, or restrictions on, the preferred stock.

        Your nominee holder must also discloseIf we issue shares of preferred stock under this prospectus, the shares will be fully paid and non-assessable and will not have, or be subject to, us certain other information received from you.any preemptive or similar rights.

        If you exercise less than allThe General Corporation Law of the subscriptionState of Delaware, the state of our incorporation, provides that the holders of preferred stock will have the right to vote separately as a class on any proposal involving fundamental changes in the rights evidenced by your subscriptionof holders of that preferred stock. This right is in addition to any voting rights that may be provided for in the applicable certificate by so indicating on your subscriptionof designation.

        The issuance of preferred stock could adversely affect the voting power, conversion or other rights certificate, the subscription agent will, if you so request, issue to you a new rights certificate evidencing the unexercised subscription rights. A new subscription rights certificate willof holders of common stock. Preferred stock could be issued quickly with terms calculated to you according to your instructions upondelay or prevent a change in control of our company or make removal of management more difficult. Additionally, the partial exerciseissuance of subscription rights only ifpreferred stock may have the subscription agent receives a properly endorsed subscription rights certificate no later thaneffect of decreasing the fifth business day prior tomarket price of common stock.

Registration and Participation Rights

        As of the expiration date of the rights offering. After that date no new subscription rights certificates will be issued. Accordingly, after such date if you exercise less than allthis prospectus, holders of your subscription rights you will lose the power to exercise your remaining subscription rights.

        Return of Excess Payment.    If you exercised your over-subscription privilege and are allocated less than all of theapproximately 9.8 million shares of our common stock for which you wishedand warrants to subscribed, your excess payment for shares that were not allocated to you will be returned to you by mail, without interest or deduction, as soon as practicable after the expiration date of the rights offering. We will deliver to the record holders who purchase shares in the rights offering certificates representing the shares of our common stock are entitled to rights with respect to the registration of those shares of common stock under the Securities Act. These registration rights require, among other things, that were purchased as soon as practicable afterif we propose to register any of our securities under the expiration dateSecurities Act, either for our own account or for the account of others, the holders of these shares are entitled to notice of the rights offeringregistration and after all pro rata allocationsare entitled to include, at our expense, their shares of common stock in the registration and adjustmentsany related underwriting, provided, among other conditions, that the underwriters may limit the number of shares to be included in the registration. In addition, the holders of these shares may require us, at our expense and subject to limitations, to file a registration statement under the Securities Act with respect to their shares of common stock. These holders have been completed.

        Transferability of Subscription Rights.    You may not transfer your subscription rights. Only you may exercise your subscription rights.

        Subscription Price.    To exercise your subscriptionwaived these registration rights you must pay in cash the subscription price of $0.64 per share of our common stock.

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Record date

        The record date for the rights offering was April 29, 2003, at 5:00 p.m., Central Daylight time.

Subscription agent

        We have appointed Wells Fargo Bank as subscription agent for the rights offering. We will pay the fees and expenses of the subscription agent. We also have agreed to indemnify the subscription agent from certain liabilities that it may incur in connection with the offerings that might be made under this registration statement.

        In addition to these registration rights, offering.holders of at least 10% of our outstanding common stock have the right to participate in securities offerings we might undertake in the future by purchasing their pro-rata share of any common or preferred stock or other securities issued by us in such an issuance. This right of participation is subject to some exclusions, including those for securities issued pursuant to

6

stock option plans, pursuant to public offerings, in connection with any stock split, stock dividend or recapitalization, in connection with equipment lease financings, in connection with corporate collaborations and in connection with acquisitions or transactions approved by our board of directors. In addition, we are prohibited from offering participation rights, rights of first refusal, rights of first offer or similar rights to anyone elase on terms more favorable than, or in preference to, the participation rights currently held by these stockholders. These holders have waived these participation rights in connection with the offerings that might be made under this registration statement.

Anti-Takeover Effects of Provisions of Delaware Law and Our Charter Documents.

        Delaware Takeover Statute.    We are subject to the provisions of Section 203 of the Delaware General Corporation Law. In general, the statute prohibits a publicly-held Delaware corporation such as Rigel from engaging in a business combination with an interested stockholder for a period of three years after the date of the transaction in which the person became an interested stockholder, unless the business combination is approved in a prescribed manner. For purposes of Section 203, a business combination includes a merger, asset sale or other transaction resulting in a financial benefit to the interested stockholder, and an interested stockholder is a person who, together with affiliates and associates, owns (or within three years prior, did own) 15% or more of the corporation's voting stock.

        Charter Documents.    Our amended and restated certificate of incorporation requires that any action required or permitted to be taken by our stockholders must be effected at a duly called annual or special meeting of stockholders and may not be effected by a consent in writing. Additionally, our amended and restated certificate of incorporation:

•

does not provide for the use of cumulative voting in the election of directors;



•

provides for a board of directors, classified into three classes of directors;



•

provides that the authorized number of directors may be changed only by resolution of our board of directors; and



•

provides for the authority of our board of directors to issue up to 10 million shares of "blank check" preferred stock and to determine the price, powers, preferences and rights of these shares, without stockholder approval.

        Our amended and restated bylaws provide that candidates for director may be nominated only by our board of directors or by a stockholder who gives written notice to us no later than 90 days prior nor earlier than 120 days prior to the first anniversary of the last annual meeting of stockholders, subject to certain exceptions. The authorized number of directors is fixed in accordance with our amended and restated certificate of incorporation. Our board of directors may appoint new directors to fill vacancies or newly created directorships. Our amended and restated bylaws also limit who may call a special meeting of stockholders.

        Delaware law and these charter provisions may have the effect of deterring hostile takeovers or delaying changes in control of our management, which could depress the market price of our common stock.

Transfer Agent and Registrar

        The transfer agent and registrar for our common stock is Wells Fargo'sFargo Bank, N. A. Its address is 161 North Concord Exchange, South St. Paul, MN 55075-1139 and its telephone number is (800) 468-9716.

7

Exercise of subscription rights

DESCRIPTION OF DEBT SECURITIES

        YouThe following description, together with the additional information we include in any applicable prospectus supplements, summarizes the material terms and provisions of the debt securities that we may exercise your subscription rightsoffer under this prospectus. While the terms we have summarized below will apply generally to any future debt securities we may offer, we will describe the particular terms of any debt securities that we may offer in more detail in the applicable prospectus supplement. The terms of any debt securities we offer under a prospectus supplement may differ from the terms we describe below. However, no prospectus supplement shall fundamentally change the terms that are set forth in this prospectus or offer a security that is not registered and described in this prospectus at the time of its effectiveness. As of the date of this prospectus, Rigel has no outstanding issuer debt.

  ��     We will issue the senior notes under the senior indenture that we will enter into with the trustee named in the senior indenture. We will issue the subordinated notes under the subordinated indenture that we will enter into with the trustee named in the subordinated indenture. We have filed forms of these documents as exhibits to the registration statement which includes this prospectus. We use the term "indentures" to refer to both the senior indenture and the subordinated indenture.

        The indentures will be qualified under the Trust Indenture Act of 1939. We use the term "debenture trustee" to refer to either the senior trustee or the subordinated trustee, as applicable.

        The following summaries of material provisions of the senior notes, the subordinated notes and the indentures are subject to, and qualified in their entirety by deliveringreference to, all the provisions of the indenture applicable to a particular series of debt securities. Except as we may otherwise indicate, the terms of the senior indenture and the subordinated indenture are identical.

General

        We will describe in each prospectus supplement the following terms relating to the subscription agent at or before 5:00 p.m. Central Daylight time on             , 2003, the expiration datea series of the rights offering:notes:

•

your properly completedthe title;



•

the principal amount being offered, and executed subscription rights certificate evidencingif a series, the total amount authorized and the total amount outstanding;



•

any limit on the amount that may be issued;



•

whether or not we will issue the series of notes in global form, the terms and who the depositary will be;



•

the maturity date;



•

whether and under what circumstances, if any, we will pay additional amounts on any debt securities held by a person who is not a United States person for tax purposes, and whether we can redeem the debt securities if we have to pay such additional amounts;



•

the annual interest rate, which may be fixed or variable, or the method for determining the rate and the date interest will begin to accrue, the dates interest will be payable and the regular record dates for interest payment dates or the method for determining such dates;



•

whether or not the notes will be secured or unsecured, and the terms of any secured debt;



•

the terms of the subordination of any series of subordinated debt;



•

the place where payments will be payable;



•

our right, if any, to defer payment of interest and the maximum length of any such deferral period;

8

•

the date, if any, after which, and the price at which, we may, at our option, redeem the series of notes pursuant to any optional or provisional redemption provisions and the terms of those subscription rights withredemptions provisions;



•

the date, if any, required signature guaranteeson which, and the price at which we are obligated, pursuant to any mandatory sinking und or analogous fund provisions or otherwise, to redeem, or at the holder's option to purchase, the series of notes and the currency or currency unit in which the debt securities are payable;



•

whether the indenture will restrict our ability to pay dividends, or will require us to maintain any asset ratios or reserves;



•

whether we will be restricted from incurring any additional indebtedness or issuing additional securities;



•

a discussion of any material or special United States federal income tax considerations applicable to the notes;



•

information describing any book-entry features;



•

provisions for a sinking fund purchase or other supplemental documentation;analogous fund, if any;



•

any provisions for payment of additional amounts for taxes and any provision for redemption, if we must pay such additional amount with respect to any debt security;



•

whether the debt securities are to be offered at a price such that they will be deemed to be offered at an "original issue discount" as defined in paragraph (a) of Section 1273 of the Internal Revenue Code;



•

the denominations in which we will issue the series of notes, if other than denominations of $1,000 and any integral multiple thereof; and



•

your paymentany other specific terms, preferences, rights or limitations of, or restrictions on, the debt securities, including any additional events of default or covenants provided with respect to the debt securities, and any terms that may be required by us or advisable under applicable laws or regulations.

Conversion or Exchange Rights

        We will set forth in fullthe prospectus supplement the terms on which a series of notes may be convertible into or exchangeable for common stock or other securities of ours. We will include provisions as to whether conversion or exchange is mandatory, at the option of the subscription price for each shareholder or at our option. We may include provisions pursuant to which the number of shares of common stock or other securities of ours that the holders of the series of notes receive would be subject to adjustment.

Consolidation, Merger or Sale

        The indentures do not contain any covenant that restricts our ability to merge or consolidate, or sell, convey, transfer or otherwise dispose of all or substantially all of our common stock subscribedassets. However, any successor to or acquirer of such assets must assume all of our obligations under the indentures or the notes, as appropriate. If the debt securities are convertible for our other securities or securities of other entities, the person with whom we consolidate or merge or to whom we sell all of our property must make provisions for the conversion of the debt securities into securities that the holders of the debt securities would have received if they had converted the debt securities before the consolidation, merger or sale.

9

Events of Default Under the Indenture

        The following are events of default under your basic subscription privilegethe indentures with respect to any series of notes that we may issue:

•

if we fail to pay interest when due and over-subscription privilege.payable and our failure continues for 90 days and the time for payment has not been extended or deferred;



•

if we fail to pay the principal, or premium, if any, when due and payable and the time for payment has not been extended or delayed;



•

if we fail to observe or perform any other covenant contained in the notes or the indentures, other than a covenant specifically relating to another series of notes, and our failure continues for 90 days after we receive notice from the debenture trustee or holders of at least 25% in aggregate principal amount of the outstanding notes of the applicable series; and



•

if specified events of bankruptcy, insolvency or reorganization occur.

        If youan event of default with respect to notes of any series occurs and is continuing, other than an event of default specified in the last bullet point above, the debenture trustee or the holders of at least 25% in aggregate principal amount of the outstanding notes of that series, by notice to us in writing, and to the debenture trustee if notice is given by such holders, may declare the unpaid principal of, premium, if any, and accrued interest, if any, due and payable immediately. If an event of default specified in the last bullet point above occurs with respect to us, the principal amount of and accrued interest, if any, of each issue of debt securities then outstanding shall be due and payable without any notice or other action on the part of the debenture trustee or any holder.

        The holders of a majority in principal amount of the outstanding notes of an affected series may waive any default or event of default with respect to the series and its consequences, except defaults or events of default regarding payment of principal, premium, if any, or interest, unless we have cured the default or event of default in accordance with the indenture. Any waiver shall cure the default or event of default.

        Subject to the terms of the indentures, if an event of default under an indenture shall occur and be continuing, the debenture trustee will be under no obligation to exercise any of its rights or powers under such indenture at the request or direction of any of the holders of the applicable series of notes, unless such holders have offered the debenture trustee reasonable indemnity. The holders of a majority in principal amount of the outstanding notes of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the debenture trustee, or exercising any trust or power conferred on the debenture trustee, with respect to the notes of that series, provided that:

•

the direction so given by the holder is not in conflict with any law or the applicable indenture; and



•

subject to its duties under the Trust Indenture Act of 1939, the debenture trustee need not take any action that might involve it in personal liability or might be unduly prejudicial to the holders not involved in the proceeding.

        A holder of the notes of any series will only have the right to institute a proceeding under the indentures or to appoint a receiver or trustee, or to seek other remedies if:

•

the holder has given written notice to the debenture trustee of a continuing event of default with respect to that series;

10

•

the holders of at least 25% in aggregate principal amount of the outstanding notes of that series have made written request, and such holders have offered reasonable indemnity to the debenture trustee to institute the proceeding as trustee; and



•

the debenture trustee does not institute the proceeding, and does not receive from the holders of a majority in aggregate principal amount of the outstanding notes of that series other conflicting directions within 90 days after the notice, request and offer.

        These limitations do not apply to a suit instituted by a holder of notes if we default in the payment of the principal, premium, if any, or interest on, the notes.

        We will periodically file statements with the debenture trustee regarding our compliance with specified covenants in the indentures.

Modification of Indenture; Waiver

        We and the debenture trustee may change an indenture without the consent of any holders with respect to specific matters, including:

•

to fix any ambiguity, defect or inconsistency in the indenture;



•

to comply with the provisions described above under "Consolidation, Merger or Sale";



•

to comply with any requirements of the Securities and Exchange Commission in connection with the qualification of any indenture under the Trust Indenture Act of 1939;



•

to add to, delete from or revise the conditions, limitations, and restrictions on the authorized amount, terms, or purposes of issue, authentication and delivery of notes, as set forth in the indenture;



•

to provide for the issuance of and establish the form and terms and conditions of the notes of any series as provided under "General" to establish the form of any certifications required to be furnished pursuant to the terms of the indenture or any series of notes, or to add to the rights of the holders of any series of notes;



•

to evidence and provide for the acceptance of appointment hereunder by a successor trustee;



•

to provide for uncertificated debt securities and to make all appropriate changes for such purpose;



•

to add to our covenants such new covenants, restrictions, conditions or provisions for the protection of the holders, and to make the occurrence, or the occurrence and the continuance, of a default in any such additional covenants, restrictions, conditions or provisions an event of default; or



•

to change anything that does not materially adversely affect the interests of any holder of notes of any series.

        In addition, under the indentures, the rights of holders of a series of notes may be changed by us and the debenture trustee with the written consent of the holders of at least a majority in aggregate principal amount of the outstanding notes of each series that is affected. However, we and the debenture trustee may only make the following changes with the consent of each holder of any outstanding notes affected:

•

extending the fixed maturity of the series of notes;



•

reducing the principal amount, reducing the rate of or extending the time of payment of interest, or reducing any premium payable upon the redemption of any notes; or

11

•

reducing the percentage of notes, the holders of which are required to consent to any amendment, supplement, modification or waiver.

Discharge

        Each indenture provides that we can elect to be discharged from our obligations with respect to one or more series of debt securities, except for obligations to:

•

register the transfer or exchange of debt securities of the series;



•

replace stolen, lost or mutilated debt securities of the series;



•

maintain paying agencies;



•

hold monies for payment in trust;



•

recover excess money held by the debenture trustee;



•

compensate and indemnify the debenture trustee; and



•

appoint any successor trustee.

        In order to exercise our rights to be discharged, we must deposit with the debenture trustee money or government obligations sufficient to pay all the principal of, any premium, if any, and interest on, the debt securities of the series on the dates payments are due.

Form, Exchange and Transfer

        We will issue the notes of each series only in fully registered form without coupons and, unless we otherwise specify in the applicable prospectus supplement, in denominations of $1,000 and any integral multiple thereof. The indentures provide that we may issue notes of a beneficial ownerseries in temporary or permanent global form and as book-entry securities that will be deposited with, or on behalf of, The Depository Trust Company or another depositary named by us and identified in a prospectus supplement with respect to that series. See "Legal Ownership of Securities" for a further description of the terms relating to any book-entry securities.

        At the option of the holder, subject to the terms of the indentures and the limitations applicable to global securities described in the applicable prospectus supplement, the holder of the notes of any series can exchange the notes for other notes of the same series, in any authorized denomination and of like tenor and aggregate principal amount.

        Subject to the terms of the indentures and the limitations applicable to global securities set forth in the applicable prospectus supplement, holders of the notes may present the notes for exchange or for registration of transfer, duly endorsed or with the form of transfer endorsed thereon duly executed if so required by us or the security registrar, at the office of the security registrar or at the office of any transfer agent designated by us for this purpose. Unless otherwise provided in the notes that the holder presents for transfer or exchange, we will make no service charge for any registration of transfer or exchange, but we may require payment of any taxes or other governmental charges.

        We will name in the applicable prospectus supplement the security registrar, and any transfer agent in addition to the security registrar, that we initially designate for any notes. We may at any time designate additional transfer agents or rescind the designation of any transfer agent or approve a change in the office through which any transfer agent acts, except that we will be required to maintain a transfer agent in each place of payment for the notes of each series.

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        If we elect to redeem the notes of any series, we will not be required to:

•

issue, register the transfer of, or exchange any notes of that series during a period beginning at the opening of business 15 days before the day of mailing of a notice of redemption of any notes that may be selected for redemption and ending at the close of business on the day of the mailing; or



•

register the transfer of or exchange any notes so selected for redemption, in whole or in part, except the unredeemed portion of any notes we are redeeming in part.

Information Concerning the Debenture Trustee

        The debenture trustee, other than during the occurrence and continuance of an event of default under an indenture, undertakes to perform only those duties as are specifically set forth in the applicable indenture. Upon an event of default under an indenture, the debenture trustee must use the same degree of care as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, the debenture trustee is under no obligation to exercise any of the powers given it by the indentures at the request of any holder of notes unless it is offered reasonable security and indemnity against the costs, expenses and liabilities that it might incur.

Payment and Paying Agents

        Unless we otherwise indicate in the applicable prospectus supplement, we will make payment of the interest on any notes on any interest payment date to the person in whose name the notes, or one or more predecessor securities, are registered at the close of business on the regular record date for the interest.

        We will pay principal of and any premium and interest on the notes of a particular series at the office of the paying agents designated by us, except that unless we otherwise indicate in the applicable prospectus supplement, we will make interest payments by check that we will mail to the holder or by wire transfer to certain holders. Unless we otherwise indicate in a prospectus supplement, we will designate the corporate trust office of the debenture trustee in the City of New York as our sole paying agent for payments with respect to notes of each series. We will name in the applicable prospectus supplement any other paying agents that we initially designate for the notes of a particular series. We will maintain a paying agent in each place of payment for the notes of a particular series.

        All money we pay to a paying agent or the debenture trustee for the payment of the principal of or any premium or interest on any notes that remains unclaimed at the end of two years after such principal, premium or interest has become due and payable will be repaid to us, and the holder of the security thereafter may look only to us for payment thereof.

Governing Law

        The indentures and the notes will be governed by and construed in accordance with the laws of the State of New York, except to the extent that the Trust Indenture Act of 1939 is applicable.

Subordination of Subordinated Notes

        The subordinated notes will be unsecured and will be subordinate and junior in priority of payment to certain of our other indebtedness to the extent described in a prospectus supplement. The subordinated indenture does not limit the amount of subordinated notes that we may issue. It also does not limit us from issuing any other secured or unsecured debt.

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DESCRIPTION OF WARRANTS

        The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the warrants that we may offer under this prospectus and the related warrant agreements and warrant certificates. While the terms summarized below will apply generally to any warrants that we may offer, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. If we indicate in the prospectus supplement, the terms of any warrants offered under that prospectus supplement may differ from the terms described below. However, no prospectus supplement shall fundamentally change the terms that are set forth in this prospectus or offer a security that is not registered and described in this prospectus at the time of its effectiveness. Specific warrant agreements will contain additional important terms and provisions and will be incorporated by reference as an exhibit to the registration statement that includes this prospectus or as an exhibit to a current report on Form 8-K.

General

        We will describe in the applicable prospectus supplement the terms of the series of warrants, including:

•

the offering price and aggregate number of warrants offered;



•

the currency for which the warrants may be purchased;



•

if applicable, the designation and terms of the securities with which the warrants are issued and the number of warrants issued with each such security or each principal amount of such security;



•

if applicable, the date on and after which the warrants and the related securities will be separately transferable;



•

in the case of warrants to purchase debt securities, the principal amount of debt securities purchasable upon exercise of one warrant and the price at, and currency in which, this principal amount of debt securities may be purchased upon such exercise;



•

in the case of warrants to purchase common stock or preferred stock, the number of shares of our common stock whoseor preferred stock, as the case may be, purchasable upon the exercise of one warrant and the price at which these shares may be purchased upon such exercise;



•

the effect of any merger, consolidation, sale or other disposition of our business on the warrant agreements and the warrants;



•

the terms of any rights to redeem or call the warrants;



•

any provisions for changes to or adjustments in the exercise price or number of securities issuable upon exercise of the warrants;



•

the dates on which the right to exercise the warrants will commence and expire;



•

the manner in which the warrant agreements and warrants may be modified;



•

federal income tax consequences of holding or exercising the warrants;



•

the terms of the securities issuable upon exercise of the warrants; and



•

any other specific terms, preferences, rights or limitations of or restrictions on the warrants.

14

        Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including:

•

in the case of warrants to purchase debt securities, the right to receive payments of principal of, or premium, if any, or interest on, the debt securities purchasable upon exercise or to enforce covenants in the applicable indenture; or



•

in the case of warrants to purchase common stock or preferred stock, the right to receive dividends, if any, or, payments upon our liquidation, dissolution or winding up or to exercise voting rights, if any.

Exercise of Warrants

        Each warrant will entitle the holder to purchase the securities that we specify in the applicable prospectus supplement at the exercise price that we describe in the applicable prospectus supplement. Unless we otherwise specify in the applicable prospectus supplement, holders of the warrants may exercise the warrants at any time up to 5:00 P.M. New York time on the expiration date that we set forth in the applicable prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void.

        Holders of the warrants may exercise the warrants by delivering the warrant certificate representing the warrants to be exercised together with specified information, and paying the required amount to the warrant agent in immediately available funds, as provided in the applicable prospectus supplement. We will set forth on the reverse side of the warrant certificate and in the applicable prospectus supplement the information that the holder of the warrant will be required to deliver to the warrant agent.

        Upon receipt of the required payment and the warrant certificate properly completed and duly executed at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement, we will issue and deliver the securities purchasable upon such exercise. If fewer than all of the warrants represented by the warrant certificate are exercised, then we will issue a new warrant certificate for the remaining amount of warrants. If we so indicate in the applicable prospectus supplement, holders of the warrants may surrender securities as all or part of the exercise price for warrants.

Governing Law

        The warrants and warrant agreements will be governed by and construed in accordance with the laws of the State of New York.

Enforceability of Rights by Holders of Warrants

        Each warrant agent will act solely as our agent under the applicable warrant agreement and will not assume any obligation or relationship of agency or trust with any holder of any warrant. A single bank or trust company may act as warrant agent for more than one issue of warrants. A warrant agent will have no duty or responsibility in case of any default by us under the applicable warrant agreement or warrant, including any duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a warrant may, without the consent of the related warrant agent or the holder of any other warrant, enforce by appropriate legal action its right to exercise, and receive the securities purchasable upon exercise of, its warrants.

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LEGAL OWNERSHIP OF SECURITIES

        We can issue securities in registered form or in the form of one or more global securities. We describe global securities in greater detail below. We refer to those persons who have securities registered in their own names on the books that we or any applicable trustee maintain for this purpose as the "holders" of those securities. These persons are the legal holders of the securities. We refer to those persons who, indirectly through others, own beneficial interests in securities that are not registered in their own names, as "indirect holders" of those securities. As we discuss below, indirect holders are not legal holders, and investors in securities issued in book-entry form or in street name will be indirect holders.

Book-Entry Holders

        We may issue securities in book-entry form only, as we will specify in the applicable prospectus supplement. This means securities may be represented by one or more global securities registered in the name of a financial institution that holds them as depositary on behalf of other financial institutions that participate in the depositary's book-entry system. These participating institutions, which are referred to as participants, in turn, hold beneficial interests in the securities on behalf of themselves or their customers.

        Only the person in whose name a security is registered is recognized as the holder of that security. Securities issued in global form will be registered in the name of the depositary or its participants. Consequently, for securities issued in global form, we will recognize only the depositary as the holder of the securities, and we will make all payments on the securities to the depositary. The depositary passes along the payments it receives to its participants, which in turn pass the payments along to their customers who are the beneficial owners. The depositary and its participants do so under agreements they have made with one another or with their customers; they are not obligated to do so under the terms of the securities.

        As a result, investors in a book-entry security will not own securities directly. Instead, they will own beneficial interests in a global security, through a bank, broker custodianor other financial institution that participates in the depositary's book-entry system or holds an interest through a participant. As long as the securities are issued in global form, investors will be indirect holders, and not holders, of the securities.

Street Name Holders

        We may terminate a global security or issue securities in non-global form. In these cases, investors may choose to hold their securities in their own names or in "street name." Securities held by an investor in street name would be registered in the name of a bank, broker or other financial institution that the investor chooses, and the investor would hold only a beneficial interest in those securities through an account he or she maintains at that institution.

        For securities held in street name, we will recognize only the intermediary banks, brokers and other financial institutions in whose names the securities are registered as the holders of those securities, and we will make all payments on those securities to them. These institutions pass along the payments they receive to their customers who are the beneficial owners, but only because they agree to do so in their customer agreements or because they are legally required to do so. Investors who hold securities in street name will be indirect holders, not holders, of those securities.

Legal Holders

        Our obligations, as well as the obligations of any applicable trustee and of any third parties employed by us or a trustee, run only to the legal holders of the securities. We do not have obligations

16

to investors who hold beneficial interests in global securities, in street name or by any other indirect means. This will be the case whether an investor chooses to be an indirect holder of a security or has no choice because we are issuing the securities only in global form.

        For example, once we make a payment or give a notice to the holder, we have no further responsibility for the payment or notice even if that holder is required, under agreements with depositary participants or customers or by law, to pass it along to the indirect holders but does not do so. Similarly, we may want to obtain the approval of the holders to amend an indenture, to relieve us of the consequences of a default or of our obligation to comply with a particular provision of the indenture or for other purposes. In such an event, we would seek approval only from the holders, and not the indirect holders, of the securities. Whether and how the holders contact the indirect holders is up to the holders.

Special Considerations For Indirect Holders

        If you hold securities through a bank, broker or other financial institution, either in book-entry form or in street name, you should check with your own institution to find out:

•

how it handles securities payments and notices;



•

whether it imposes fees or charges;



•

how it would handle a request for the holders' consent, if ever required;



•

whether and how you can instruct it to send you securities registered in your own name so you can be a holder, if that is permitted in the future;



•

how it would exercise rights under the securities if there were a default or other event triggering the need for holders to act to protect their interests; and



•

if the securities are in book-entry form, how the depositary's rules and procedures will affect these matters.

Global Securities

        A global security is a security that represents one or any other number of individual securities held by a depositary. Generally, all securities represented by the same global securities will have the same terms.

        Each security issued in book-entry form will be represented by a global security that we deposit with and register in the name of a financial institution or its nominee that we select. The financial institution that we select for this purpose is called the depositary. Unless we specify otherwise in the applicable prospectus supplement, The Depository Trust Company, New York, New York, known as DTC, will be the depositary for all securities issued in book-entry form.

        A global security may not be transferred to or registered in the name of anyone other than the depositary, its nominee or a successor depositary, unless special termination situations arise. We describe those situations below under "Special Situations When a Global Security Will Be Terminated." As a result of these arrangements, the depositary, or its nominee, will be the sole registered owner and holder of all securities represented by a global security, and investors will be permitted to own only beneficial interests in a global security. Beneficial interests must be held by means of an account with a broker, bank or other nominee, you should instruct your broker, custodian bankfinancial institution that in turn has an account with the depositary or other nominee to exercise your subscription rights and deliver all documents and payment on your behalf prior to 5:00 p.m. Central Daylight time on            , 2003, the expiration date of the rights offering.

        Your subscription rightswith another institution that does. Thus, an investor whose security is represented by a global security will not be considered exercised unless the subscription agent receives from you, your broker, custodian or nominee, as the case may be, alla holder of the required documentssecurity, but only an indirect holder of a beneficial interest in the global security.

        If the prospectus supplement for a particular security indicates that the security will be issued in global form only, then the security will be represented by a global security at all times unless and your full subscription price payment prioruntil

17

the global security is terminated. If termination occurs, we may issue the securities through another book-entry clearing system or decide that the securities may no longer be held through any book-entry clearing system.

Special Considerations For Global Securities

        As an indirect holder, an investor's rights relating to 5:00 p.m. Central Daylight time on            , 2003,a global security will be governed by the expiration dateaccount rules of the rights offering.

Once you exercise your subscription rights, you cannot revoke your subscription. In order to exercise your subscription rights, you must exercise them before they expire.

Method of payment

        Your paymentinvestor's financial institution and of the subscription price mustdepositary, as well as general laws relating to securities transfers. We do not recognize an indirect holder as a holder of securities and instead deal only with the depositary that holds the global security.

        If securities are issued only in the form of a global security, an investor should be made in U.S. dollars foraware of the full number of shares of our common stock for which you are subscribing by either:following:

•

checkAn investor cannot cause the securities to be registered in his or her name, and cannot obtain non-global certificates for his or her interest in the securities, except in the special situations we describe below;



•

An investor will be an indirect holder and must look to his or her own bank or broker for payments on the securities and protection of his or her legal rights relating to the securities, as we describe above;



•

An investor may not be able to sell interests in the securities to some insurance companies and to other institutions that are required by law to own their securities in non-book-entry form;



•

An investor may not be able to pledge his or her interest in a global security in circumstances where certificates representing the securities must be delivered to the lender or other beneficiary of the pledge in order for the pledge to be effective;



•

The depositary's policies, which may change from time to time, will govern payments, transfers, exchanges and other matters relating to an investor's interest in a global security. We and any applicable trustee have no responsibility for any aspect of the depositary's actions or for its records of ownership interests in a global security. We and the trustee also do not supervise the depositary in any way;



•

The depositary may, and we understand that DTC will, require that those who purchase and sell interests in a global security within its book-entry system use immediately available funds, and your broker or bank draft drawn uponmay require you to do so as well; and



•

Financial institutions that participate in the depositary's book-entry system, and through which an investor holds its interest in a U.S. bankglobal security, may also have their own policies affecting payments, notices and other matters relating to the securities. There may be more than one financial intermediary in the chain of ownership for an investor. We do not monitor and are not responsible for the actions of any of those intermediaries.

Special Situations When A Global Security Will Be Terminated

        In a few special situations described below, the global security will terminate and interests in it will be exchanged for physical certificates representing those interests. After that exchange, the choice of whether to hold securities directly or postal, telegraphicin street name will be up to the investor. Investors must consult their own banks or express money order payablebrokers to Wells Fargo Bank,find out how to have their interests in securities transferred to their own name, so that they will be direct holders. We have described the rights of holders and street name investors above.

18

        The global security will terminate when the following special situations occur:

•

if the depositary notifies us that it is unwilling, unable or no longer qualified to continue as subscription agent;depositary for that global security and we do not appoint another institution to act as depositary within 90 days;



•

if we notify any applicable trustee that we wish to terminate that global security; or



•

wire transferif an event of immediately available fundsdefault has occurred with regard to securities represented by that global security and has not been cured or waived.

        The prospectus supplement may also list additional situations for terminating a global security that would apply only to the account maintainedparticular series of securities covered by the subscription agentprospectus supplement. When a global security terminates, the depositary, and not we or any applicable trustee, is responsible for such purpose at            , ABA No.            , Account No.             (marked: "Rigel Pharmaceuticals, Inc. Subscription").deciding the names of the institutions that will be the initial direct holders.

Receipt of PaymentPLAN OF DISTRIBUTION

        Your paymentWe may sell the securities through underwriters or dealers, through agents, or directly to one or more purchasers. One or more prospectus supplements will describe the terms of the subscription price will be deemed to have been received byoffering of the subscription agent only upon:securities, including:

•

clearancethe name or names of any uncertified check;underwriters, if any;



•

receipt by the subscription agentpurchase price of any certified check or bank draft drawn upon a U.S. bank or any postal, telegraphic or express money order; orthe securities and the proceeds we will receive from the sale;



•

receipt of collected fundsany over-allotment options under which underwriters may purchase additional securities from us;



•

any agency fees or underwriting discounts and other items constituting agents' or underwriters' compensation;



•

any initial public offering price;



•

any discounts or concessions allowed or reallowed or paid to dealers; and



•

any securities exchange or market on which the securities may be listed.

        Only underwriters named in the subscription agent's account designated above.

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Clearance of uncertified checks

        You should note that funds paid by uncertified personal checks may take at least five business days to clear. If you wish to pay the subscription price by an uncertified personal check, we urge you to make payment sufficiently in advanceprospectus supplement are underwriters of the timesecurities offered by the subscription rights expire to ensure that your payment is received and clears by that time. We urge you to consider using a certified or cashier's check, money order or wire transfer of funds to avoid missing the opportunity to exercise your subscription rights.

Delivery of subscription materials and paymentprospectus supplement.

        You should deliverIf underwriters are used in the subscription rights certificatesale, they will acquire the securities for their own account and paymentmay resell them from time to time in one or more transactions at a fixed public offering price. The obligations of the subscription price, as well as any Nominee Holder Certifications, Notices of Guaranteed Delivery and DTC Participant Over-Subscription Forms,

if by mail, hand or overnight courier to:
Wells Fargo Bank
161 North Concord Exchange
South St. Paul, MN 55075
Attn: Corporate Actions, Rigel Pharmaceuticals, Inc. Rights Offering

        You may call the subscription agent at (800) 468-9716.

        Your delivery to another address or by any method other than as set forth above will not constitute valid delivery.

Calculation of subscription rights exercised

        If you do not indicate the number of subscription rights being exercised, or do not forward full payment of the total subscription price for the number of subscription rights that you indicate are being exercised, then you will be deemed to have exercised the basic subscription privilege with respect to the maximum number of subscription rights that may be exercised for the aggregate subscription price payment you delivered to the subscription agent. If your aggregate subscription price payment is greater than the amount you owe for your subscription, you will be deemed to have exercised the full basic subscription privilege and the over-subscription privilegeunderwriters to purchase the maximum number of shares of our common stock with your overpayment. If we do not apply your full subscription price payment to your purchase of shares of our common stock, we will return the excess amount to you by mail without interest or deduction as soon as practicable after the expiration date of the rights offering.

Your fundssecurities will be held by the subscription agent until shares of our common stock are issued

        The subscription agent will hold your payment of the subscription price in a segregated account with other payments received from holders of subscription rights until we issue to you your shares of our common stock upon completion of the rights offering.

        If you exercised your over-subscription privilege and are allocated less than all of the shares of our common stock for which you wished to subscribe, the excess funds you paid for shares of our common stock that are not allocated to you will be returned by mail without interest or deduction as soon as practicable after the expiration date of the subscription rights.

Medallion guarantee may be required

        Your signature on each subscription rights certificate must be guaranteed by an eligible institution (a member firm of a registered national securities exchange or a member of the National Association

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of Securities Dealers, Inc. or a commercial bank or trust company having an office or correspondent in the United States), subject to standards and procedures adopted by the subscription agent, unless

•

your subscription rights certificate provides that the shares of our common stock you subscribed for are to be delivered to you as record holder of those subscription rights; or



•

you are an eligible institution.

Notice to beneficial holders

        If you are a broker, a trustee or a depositary for securities who holds shares of our common stock for the account of others on April 29, 2003 (a "nominee record date holder"), you should notify the respective beneficial owners of such shares of the subscription rights as soon as possible to find out such beneficial owners' intentions with respect to exercising their subscription rights. You should obtain instructions from the beneficial owner with respect to the subscription rights, asconditions set forth in the instructionsapplicable underwriting agreement. We may offer the securities to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. Subject to certain conditions, the underwriters will be obligated to purchase all the securities of the series offered by the prospectus supplement. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealers may change from time to time. We may use underwriters with whom we have provideda material relationship. We will describe in the prospectus supplement, naming the underwriter, the nature of any such relationship.

        We may sell securities directly or through agents we designate from time to you for your distribution to beneficial owners. Iftime. We will name any agent involved in the beneficial owner so instructs, you should completeoffering and sale of securities and we will describe any commissions we will pay the appropriate subscription rights certificates and submit them toagent in the subscriptionprospectus supplement.

        Unless the prospectus supplement states otherwise, our agent with the proper payment. If you hold shares of our common stockwill act on a best-efforts basis for the account(s)period of more than one beneficial owner, youits appointment. However, no prospectus supplement shall fundamentally change the terms that are set forth in this prospectus or offer a security that is not registered and described in this prospectus at the time of its effectiveness.

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        We may exerciseauthorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the number of subscription rights to which all such beneficial ownerspublic offering price set forth in the aggregate otherwise would have been entitled had they been direct record holdersprospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of our common stock onthese contracts in the record date, provided that you, as a nominee record holder, make a proper showing toprospectus supplement.

        We may provide agents and underwriters with indemnification against certain civil liabilities, including liabilities under the subscription agent by submitting the form entitled "Nominee Holder Certification" that we will provide to you with your rights offering materials. If you did not receive this form, you should contact the subscription agent to request a copy.

Beneficial owners

        If you are a beneficial owner of shares of our common stockSecurities Act, or will receive your subscription rights through a broker, custodian bank or other nominee, we will ask your broker, custodian bank or other nominee to notify you of the rights offering. If you wish to exercise your subscription rights, you will need to have your broker, custodian bank or other nominee act for you. If you hold certificates of our common stock directly and would prefer to have your broker, custodian bank or other nominee act for you, you should contact your nominee and request it to effect the transactions for you. To indicate your decisioncontribution with respect to your subscription rights, you should completepayments that the agents or underwriters may make with respect to such liabilities. Agents and returnunderwriters may engage in transactions with, or perform services for, us in the ordinary course of business.

        All securities we offer, other than common stock, will be new issues of securities with no established trading market. Any underwriters may make a market in these securities, but will not be obligated to your broker, custodian bank or other nomineedo so and may discontinue any market making at any time without notice. We cannot guarantee the form entitled "Beneficial Owners Election Form." You should receive this form from your broker, custodian bank or other nomineeliquidity of the trading markets for any securities.

        Any underwriter may engage in overallotment, stabilizing transactions, short covering transactions and penalty bids in accordance with Regulation M under the other rightsSecurities Exchange Act of 1934. Overallotment involves sales in excess of the offering materials. If you wishsize, which create a short position. Stabilizing transactions permit bids to obtain a separate subscription rights certificate, you should contactpurchase the nomineeunderlying security so long as soon as possible and request that a separate subscription rights certificate be issued to you. You should contact your broker, custodian bank or other nominee if youthe stabilizing bids do not receive this form, but you believe you are entitled to participateexceed a specified maximum. Short covering transactions involve purchases of the securities in the rights offering. Weopen market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are not responsible if you do not receivepurchased in a covering transaction to cover short positions. Those activities may cause the form from your broker, custodian bank or nominee or if you receiveprice of the securities to be higher than it without sufficient time to respond.

Instructions for completing your subscription rights certificate

        You should read and followwould otherwise be. If commenced, the instructions accompanyingunderwriters may discontinue any of the subscription rights certificates carefully. If you want to exercise your subscription rights, you must send your subscription rights certificates to the subscription agent.You should not send the subscription rights certificates to Rigel.activities at any time.

        YouAny underwriters who are responsible forqualified market makers on The Nasdaq National Market may engage in passive market making transactions in the methodsecurities on The Nasdaq National Market in accordance with Rule 103 of delivery of your subscription rights certificate(s) with your subscription price payment toRegulation M under the subscription agent. If you send your subscription rights certificate(s) and subscription price payment by mail, we recommend that you send them by registered mail, properly insured, with return receipt requested. You should allow a sufficient number of days to ensure delivery toExchange Act during the subscription agent and clearance of paymentbusiness day prior to the timepricing of the subscription rights expire.

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Because uncertified personal checks may takeoffering, before the commencement of offers or sales of the securities. Passive market makers must comply with applicable volume and price limitations and must be identified as passive market makers. In general, a passive market maker must display its bid at least five business days to clear, we strongly urge you to pay, or arrange for payment, by means of certified or cashier's check, money order or wire transfer of funds.

Determinations regarding the exercise of your subscription rights

        We will decide all questions concerning the timeliness, validity, form and eligibility of your exercise of subscription rights. Our decisions will be final and binding. We, in our sole discretion, may waive any defect or irregularity, or permit a defect or irregularity to be corrected within such time as we may determine. We will not be required to make uniform determinations in all cases. We may reject the exercise of any of your subscription rights because of any defect or irregularity. Your subscription will not be deemed to have been received or accepted until all irregularities have been waived by us or cured by you within such time we decide, in our sole discretion.

        Neither we nor the subscription agent will be under any duty to notify you of a defect or irregularity in connection with your submission of subscription rights certificates. We will not be liable for failing to give you such notice. We reserve the right to reject your exercise of subscription rights if your exercise isprice not in accordance with the termsexcess of the rights offering or in proper form. We will also not accept your exercisehighest independent bid for such security; if all independent bids are lowered below the passive market maker's bid, however, the passive market maker's bid must then be lowered when certain purchase limits are exceeded.

        In compliance with guidelines of subscription rights if our issuance of shares of our common stock pursuant to your exercise could be deemed unlawful or materially burdensome.

Regulatory limitation

        We will not be required to issue shares of our common stock pursuant to the rights offering to you if, in our opinion, you would be required to obtain prior clearance or approval from any state or federal regulatory authorities to own or control such shares if, at the time the subscription rights expire, you have not obtained such clearance or approval.

Guaranteed delivery procedures

        If you wish to exercise your subscription rights, but you do not have sufficient time to deliver the subscription rights certificates evidencing your subscription rights to the subscription agent on or before the time the subscription rights expire, you may exercise your subscription rights by the following guaranteed delivery procedures:

•

deliver to the subscription agent on or prior to the rights offering expiration date your subscription price payment in full for each share of our common stock you subscribed for under your basic subscription privilege and your over-subscription privilege (in the manner set forth in "—Exercise of Subscription Rights" beginning on page 22);



•

deliver to the subscription agent on or prior to the rights offering expiration date the form entitled "Notice of Guaranteed Delivery," substantially in the form provided with the "Instructions as to Use of Rigel Pharmaceuticals, Inc. Subscription Rights Certificates" distributed with your subscription rights certificates; and



•

deliver the properly completed subscription rights certificate evidencing the subscription rights being exercised and the related nominee holder certification, if applicable, with any required signature guarantee, to the subscription agent within three Nasdaq National Market trading days following the date the Notice of Guaranteed Delivery was delivered to the subscription agent.

        Your Notice of Guaranteed Delivery must be substantially in the form provided with the Instructions as to Use of Rigel Pharmaceuticals, Inc. Subscription Rights Certificates distributed to you with your subscription rights certificate. Your Notice of Guaranteed Delivery must come from an eligible institution (a member firm of a registered national securities exchange or a member of the

25

National Association of Securities Dealers, Inc. or a commercial bankNASD, the maximum consideration or trust company having an officediscount to be received by any NASD member or correspondent inindependent broker dealer may not exceed 8% of the United States).

        In your Noticeaggregate amount of Guaranteed Delivery you must state:

•

your name;



•

the number of subscription rights represented by your subscription rights certificates, the number of shares of our common stock you are subscribing forsecurities offered pursuant to the basic subscription privilege and the number of the shares of our common stock, if any, you are subscribing for pursuant to the over-subscription privilege; and



•

your guarantee that you will deliver to the subscription agent any subscription rights certificates evidencing the subscription rights you are exercising within three Nasdaq National Market trading days following the date the subscription agent receives your Notice of Guaranteed Delivery.

        You may deliver the Notice of Guaranteed Delivery to the subscription agent in the same manner as the subscription rights certificate at the address set forth in "—Delivery of Subscription Materials and Payment" beginning on page 23. You may alternatively transmit the Notice of Guaranteed Delivery to the subscription agent by facsimile transmission at (    )            . To confirm facsimile deliveries, you may call            .

        The subscription agent will send you additional copies of the form of Notice of Guaranteed Delivery if you need them. Please call the subscription agent at (    )             to request any copies of the form of Notice of Guaranteed Delivery.

Questions about exercising subscription rights

        You may direct any questions or require assistance regarding the method of exercising your subscription rights, additional copies of this prospectus, the Instructions as to the Use of Rigel Pharmaceuticals, Inc. Subscription Rights Certificates, the Nominee Holder Certification, the Notice of Guaranteed Delivery or other subscription documents referred to herein, to            at the following telephone number and address.

---





---


(            )

No revocation

        Once you have exercised your basic subscription privilege and/or over-subscription privilege, you may not revoke your exercise. Subscription rights not exercised prior to the expiration date of the rights offering will expire and will have no value.

Procedures for DTC participants

        We expect that your exercise of your basic subscription privilege and your over-subscription privilege may be made through the facilities of The Depository Trust Company ("DTC"). If your subscription rights are held of record through DTC, you may exercise your basic subscription privilege and your over-subscription privilege by instructing DTC to transfer your subscription rights from your account to the account of the subscription agent, together with certification as to the aggregate number of subscription rights you are exercising and the number of shares of our common stock you are subscribing for under your basic subscription privilege and your over-subscription privilege, if any, and

26

your subscription price payment for each share of our common stock that you subscribed for pursuant to your basic subscription privilege and your over-subscription privilege.

Determination of subscription price

        We believe that the $0.64 per share subscription price meets our objective of raising the maximum amount of net proceeds while providing you with an opportunity to make an additional investment in our common stock. Our board of directors chose the $0.64 per share subscription price to equal the per share price paid by the private placement investors in the private placement.

        The $0.64 per share subscription price should not be considered an indication of the actual value of Rigel or of our common stock. We cannot assure you that the market price of our common stock will not decline during or after the rights offering. We also cannot assure you that you will be able to sell shares of common stock purchased during the rights offering at a price equal to or greater than $0.64 per share. We urge you to obtain a current quote for our common stock before exercising your subscription rights. On            , 2003, the closing price of our common stock was $    . Our common stock is traded on the Nasdaq National Market under the symbol "RIGL."

No recommendations to subscription rights holders

        An investment in shares of our common stock must be made according to each investor's evaluation of its own best interests and after considering all of the information in this prospectus, including the "Risk Factors" section of this prospectus and all of the information incorporated by reference in this prospectus. None of our board of directors, our officers or any other person are making any recommendations as to whether or not you should exercise your subscription rights. You should make your decision based on your own assessment of your best interests.applicable prospectus supplement.

Non-U.S. and certain other stockholders

        We will not mail subscription rights certificates to record date holders whose addresses are outside the United States or who have an army post office or fleet post office address. Instead, we will have the subscription agent hold such subscription rights certificates for such holders' accounts. To exercise their subscription rights, such holders must notify the subscription agent prior to 11:00 a.m. Central Daylight time on   ��                    , 2003, three business days prior to the expiration date, and must establish to the satisfaction of the subscription agent that such exercise is permitted under applicable law.

Issuance of common stock

        The subscription agent will issue to you certificates representing shares of our common stock you purchase pursuant to the rights offering as soon as practicable after the time the subscription rights expire.

        Your payment of the subscription price will be retained by the subscription agent, and will not be delivered to us, until your subscription is accepted and you are issued your stock certificates. We will not pay you any interest on funds paid to the subscription agent, regardless of whether such funds are applied to the subscription price or returned to you. You will have no rights as a stockholder of Rigel with respect to shares of our common stock subscribed for until certificates representing such shares are issued to you. Unless otherwise instructed in the subscription rights certificates, your certificates for shares issued pursuant to your exercise of subscription rights will be registered in your name.

        If the rights offering is not completed for any reason, the subscription agent will promptly return, without interest, all funds received by it.

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Shares of common stock outstanding after the rights offering

        Assuming we issue all of the shares of our common stock offered in the rights offering and the private placement is consummated, approximately 133,876,003 shares of our common stock will be issued and outstanding after the expiration of the rights offering (without giving effect to the exercise of the warrants issued in the private placement). Based on the 118,251,003 shares of our common stock outstanding as of June    , 2003 (immediately after the private placement), our issuance of shares in the rights offering would result in a     % increase in the number of outstanding shares of our common stock.

Other matters

        We are not making the rights offering in any state or other jurisdiction in which it is unlawful to do so. We will not sell or accept an offer to purchase our common stock from you if you are a resident of any such state or other jurisdiction. We may delay the commencement of the rights offering in certain states or other jurisdictions in order to comply with the laws of such states or other jurisdictions. We do not expect that there will be any changes in the terms of the rights offering. However, we may decide, in our sole discretion, not to modify the terms of the rights offering as may be requested by certain states or other jurisdictions. If that happens and you are a resident of that state, you will not be eligible to participate in the rights offering.

CERTAIN UNITED STATES FEDERAL INCOME TAX CONSEQUENCES

        The following discussion is a summary of certain United States federal income tax consequences of the rights offering to holders of our common stock that hold such stock as a capital asset for United States federal income tax purposes. This discussion is based on laws, regulations, rulings and decisions in effect on the date hereof, all of which are subject to change (possibly with retroactive effect) and to differing interpretations. This discussion applies only to holders that are U.S. persons and does not address all aspects of United States federal income taxation that may be relevant to holders in light of their particular circumstances or to holders who may be subject to special tax treatment under the Internal Revenue Code, including, without limitation, holders of our warrants, holders who are dealers in securities or foreign currency, foreign persons, insurance companies, tax-exempt organizations, banks, financial institutions, broker-dealers, holders who hold common stock as part of a hedge, straddle, conversion or other risk reduction transaction, or who acquired common stock pursuant to the exercise of compensatory stock options or otherwise as compensation.

        Moreover, this summary does not address the tax consequences of the rights offering under state, local or foreign tax laws. ACCORDINGLY, YOU SHOULD CONSULT YOUR OWN TAX ADVISORS TO DETERMINE THE SPECIFIC TAX CONSEQUENCES OF THE RIGHTS OFFERING TO YOU.

Issuance of the subscription rights

        If you hold Rigel common stock on the record date, you should not recognize taxable income upon the receipt of the subscription rights.

        In general, a distribution by a corporation to its stockholders of subscription rights to acquire stock of the distributing corporation is not taxable. An exception to this general rule applies in the case of a distribution which constitutes a disproportionate distribution with respect to any class or classes of stock of the corporation. A distribution of stock rights constitutes a disproportionate distribution if it is a part of a distribution or a series of distributions (including deemed distributions) that has the effect of (1) the receipt of property (including cash) by some stockholders and (2) an increase in the proportionate interests of other stockholders in the assets or earnings and profits of the distributing corporation.

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        The distribution of the subscription rights to all stockholders except the private placement investors should not constitute a disproportionate distribution taxable as a dividend since (1) the private placement was made with the expectation of promptly thereafter making the distribution of subscription rights to the remaining stockholders, and the subscription rights may be exercised on the same economic terms as the private placement, (2) there is a single class of stock outstanding, and (3) there has not been nor is there expected to be any property distributions to any stockholder in connection with the distribution of subscription rights.

        We intend to treat the distribution of subscription rights as a nontaxable distribution. If the Internal Revenue Service were to take a contrary position with respect to this matter, by deeming the distribution of subscription rights to constitute a taxable distribution, a person receiving a right would recognize a dividend, taxable as ordinary income, in an amount equal to the fair market value of the right received, but only to the extent of Rigel's current and accumulated earnings and profits, if any. To the extent the deemed distribution exceeds such current and accumulated earnings and profits, any excess would be treated first as a nontaxable recovery of adjusted tax basis in your Rigel common stock with respect to which the right was distributed and then as gain from the sale or exchange of your Rigel common stock. Your tax basis in a right received in a taxable distribution would equal the fair market value of the right as of the date of distribution of the right. Your holding period in the right would begin on the day following the date of distribution of the right.

        The following discussion assumes that the distribution of the subscription rights will be treated as a nontaxable distribution.

Basis and holding period of the subscription rights

        Generally, if you hold Rigel common stock on the record date, your basis in the subscription rights you receive will be zero. If, however, (1) the fair market value of the subscription rights on the date we issue the subscription rights is 15% or more of the fair market value (on that same date) of our common stock, or (2) you properly elect under Section 307 of the Internal Revenue Code in your federal income tax return to allocate part of the basis of your Rigel common stock to the subscription rights, then your basis in your shares of Rigel common stock will be allocated between your Rigel common stock and the subscription rights in proportion to the fair market values of each on the date we issue the subscription rights. We have not obtained an independent appraisal of the valuation of the subscription rights and, therefore, each stockholder individually must determine how Internal Revenue Code Section 307 will apply in that stockholder's particular situation.

        The holding period of your subscription rights will include your holding period (as of the date of issuance) of the Rigel common stock with respect to which we distributed the subscription rights to you.

Expiration of the subscription rights

        If your basis in your subscription rights is zero, and you allow your subscription rights to expire unexercised, you will not recognize any gain or loss.

        If you have a basis in your subscription rights and you allow your subscription rights to expire unexercised, you will recognize a loss equal to the basis of those subscription rights. Any loss you recognize on the expiration of your subscription rights will be a capital loss if the Rigel common stock obtainable by you upon exercise of the subscription rights would be a capital asset.

Exercise of the subscription rights, basis and holding period of acquired shares

        You will not recognize any gain or loss upon the exercise of your subscription rights. Your basis in each share of Rigel common stock you acquire through exercise of your subscription rights will equal

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the sum of the subscription price you paid to exercise your subscription rights and your basis, if any, in the subscription rights. Your holding period for the Rigel common stock you acquire through exercise of your subscription rights will begin on the date you exercise your subscription rights.

Sale or exchange of common stock

        If you sell or exchange shares of Rigel common stock, you will generally recognize gain or loss on the transaction. The gain or loss you recognize will be equal to the difference between the amount you realize on the transaction and your basis in the shares you sell. Such gain or loss generally will be capital gain or loss so long as you held the shares as a capital asset at the time of the sale or exchange. Gain or loss from a capital asset held for more than one year will generally be taxable as long term capital gain or loss.

Information reporting and backup withholding

        You may be subject to backup withholding with respect to the rights offering. However, you will not be subject to backup withholding if you: (1) are a corporation or fall within certain other exempt categories and, when required, demonstrate that fact; or (2) provide a correct taxpayer identification number and certify under penalties of perjury that your taxpayer identification number is correct and that you are not subject to backup withholding because you previously failed to report all dividends and interest income.

        Any amount withheld under these rules will be credited against your federal income tax liability. We may require you to establish your exemption from backup withholding or make other arrangements with respect to the payment of backup withholding.

THIS SUMMARY IS INCLUDED FOR GENERAL INFORMATION ONLY. YOU SHOULD CONSULT YOUR OWN TAX ADVISORS REGARDING THE CONSEQUENCES OF THE RIGHTS OFFERING TO YOUR PARTICULAR TAX SITUATION, INCLUDING STATE AND LOCAL INCOME AND OTHER TAX LAWS.

PLAN OF DISTRIBUTION

        We are making the rights offering directly to you, the holders of our common stock. We have not employed any brokers, dealers or underwriters in connection with the rights offering and will not pay any underwriting commissions, fees or discounts in connection with the rights offering. Some of our directors or officers may assist in the rights offering. These individuals will not receive any commissions or compensation other than their normal directors' fees or employment compensation.

        We will bear all costs, expenses and fees in connection with the rights offering. We will pay the subscription agent a fee of $                        . We estimate that our total expenses in connection with the rights offering, including fees to the subscription agent, will be $                        .

LEGAL MATTERS

        The validity of the shares of common stocksecurities being offered in the rights offering and the tax matters discussed in this prospectushereby will be passed upon for us by Cooley Godward LLP, Palo Alto, California. As of the date of this prospectus, certain partners and associates of Cooley Godward LLP own an aggregate of approximately 17,400 shares of our common stock, either individually or through investment partnerships.

EXPERTS

        The financial statements of Rigel Pharmaceuticals, Inc. appearing in Rigel Pharmaceuticals, Inc.'s Annual Report (Form 10-K), as amended, for the year ended December 31, 2002, as amended, have been audited by Ernst & Young LLP, independent auditors, as set forth in their report thereon included therein and

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incorporated herein by reference. Such financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

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        With respect to the unaudited condensed consolidated interim financial information for the three-month periods ended March 31, 2003 and March 31, 2002, the three and six-month periods ended June 30, 2003 and June 30, 2002 and the three and nine-month periods ended September 30, 2003 and September 30, 2002 incorporated by reference in this Prospectus,prospectus, Ernst & Young LLP havehas reported that they haveit has applied limited procedures in accordance with professional standards for a review of such information. However, theirits separate report,reports, included in Rigel Pharmaceuticals, Inc.'s Quarterly ReportReports on Form 10-Q for the quarterquarters ended March 31, 2003, June 30, 2003 and September 30, 2003, and incorporated herein by reference, statesstate that theyit did not audit and they doit does not express an opinion on that interim financial information. Accordingly, the degree of reliance on their reportits reports on such information should be restricted considering the limited nature of the review procedures applied. The independent auditors are not subject to the liability provisions of Section 11 of the Securities Act of 1933 (the "Act") for their reportits reports on the unaudited interim financial information because that report isthose reports are not a "report""reports" or a "part" of the Registration Statement prepared or certified by the auditors within the meaning of Sections 7 and 11 of the Securities Act.

WHERE YOU CAN FIND MORE INFORMATION

        We are a reporting company and file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission. We have filed with the Securities and Exchange Commission a registration statement on Form S-3 under the Securities Act with respect to the shares of our common stock to be issued in the rights offering.and preferred stock, debt securities and/or warrants we are offering under this prospectus. This prospectus does not contain all of the information set forth in the registration statement and the exhibits to the registration statement. For further information with respect to us and the securities offeredwe are offering under this prospectus, we refer you to the registration statement and the exhibits and schedules filed as a part of the registration statement. You may read and copy the registration statement, as well as our reports, proxy statements and other information, at the Securities and Exchange Commission's public reference room at Room 1024, 450 Fifth Street, N.W., Washington, D.C. 20549. You can request copies of these documents by writing to the Securities and Exchange Commission and paying a fee for the copying cost. Please call the Securities and Exchange Commission at 1-800-SEC-0330 for more information about the operation of the public reference rooms.room. Our Securities and Exchange Commission filings are also available at the Securities and Exchange Commission's web site at http://www.sec.gov. In addition, you can read and copy our Securities and Exchange Commission filings at the office of the National Association of Securities Dealers, Inc. at 1735 K Street, N.W., Washington, D.C. 20006.

        The Securities and Exchange Commission allows us to "incorporate by reference" the information contained in documents that we file with them,it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to bean important part of this prospectus. Information in this prospectus supersedes information incorporated by reference that we filed with the Securities and Exchange Commission prior to the date of this prospectus, while information that we file later with the Securities and Exchange Commission will automatically update and supersede this information. We incorporate by reference into this registration statement and prospectus the documents listed below and any future filings we will make with the Securities and Exchange Commission under Sections 13(a), 13(c), 14 andor 15(d) of the Securities Exchange Act after the date of 1934.the initial registration statement but prior to effectiveness of the registration statement and after the date of this prospectus but prior to the termination of the offering of the securities covered by this prospectus.

        We incorporate21

        The following documents filed with the Securities and Exchange Commission are incorporated by reference intoin this prospectusprospectus:

1.

Our annual report on Form 10-K, as amended, for the following documents, which contain important information about usyear ended December 31, 2002, filed with the Securities and our business and financial results:

Exchange Commission on March 31, 2003;

•2.

our Quarterly ReportOur quarterly report on Form 10-Q for the quarter ended March 31, 2003;

•

our Annual Report on Form 10-K for the fiscal year ended December 31, 2002, as amended on May 8, 2003;

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•

our proxy statement for the 2003, annual meeting of stockholders, filed with the Securities and Exchange Commission on May 19,15, 2003;



3.

Our quarterly report on Form 10-Q for the quarter ended June 30, 2003, filed with the Securities and Exchange Commission on August 14, 2003;



4.

Our quarterly report on Form 10-Q for the quarter ended September 30, 2003, filed with the Securities and Exchange Commission on November 14, 2003;



5.

Our current report of Form 8-K, filed with the Securities and Exchange Commission on May 2, 2003;



6.

Our current report of Form 8-K, filed with the Securities and Exchange Commission on June 24, 2003;



7.

Our current report of Form 8-K, filed with the Securities and Exchange Commission on June 27, 2003;



8.

Our current report of Form 8-K, filed with the Securities and Exchange Commission on August 4, 2003; and



•9.

theThe description of our common stock set forth in our registration statement on Form 8-A, filed with the Securities and Exchange Commission on October 3, 2000.2000, including any amendments or reports filed for the purposes of updating this description.

        We may file additional documents with the Securities and Exchange Commission pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934 on or after the date of this prospectus and before the expiration of the rights offering. The Securities and Exchange Commission allows us to incorporate by reference into this prospectus such documents. You should consider any statement contained in this prospectus (or in a document incorporated into this prospectus) to be modified or superseded to the extent that a statement in a subsequently filed document modifies or supersedes such statement.

        We will furnish without charge to you, on written or oral request, a copy of any or all of the documents incorporated by reference, including exhibits to these documents. You should direct any requests for documents to Rigel Pharmaceuticals, Inc., Attention: Corporate Secretary, 1180 Veterans Blvd., South San Francisco, California 94080, telephone:94080. Our phone number is (650) 624-1100.

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15,625,000 Shares

Rigel Pharmaceuticals, Inc.

Common Stock

PROSPECTUS

                        , 2003

PART II

INFORMATION NOT REQUIRED IN THE PROSPECTUS

Item 14.ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTIONOther Expenses of Issuance and Distribution

        The following table sets forth the estimated costs and expenses, other than the underwriting discounts and commissions, payable by the registrant in connection with the issuance and distributionoffering of the securities offered hereby.being registered. All the amounts shown are estimates, except for the registration fee.

*

To be provided by Amendment

Item 15.ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERSIndemnification of Officers and Directors

        As permitted by Delaware law, ourOur amended and restated certificate of incorporation provides that no director will be personally liablewe must indemnify our directors to us or our stockholdersthe fullest extent under applicable law. Pursuant to Delaware law, this includes elimination of liability for monetary damages for breach of the directors' fiduciary duty as a director, exceptof care to Rigel and its stockholders. However, our directors may be personally liable for liability:

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for any breach of duty of loyalty to us or to our stockholders;



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for acts or omissions not in good faith or that involve intentional misconduct or a knowing violation of law;



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for unlawful payment of dividends or unlawful stock repurchases or redemptions under Section 174 of the Delaware General Corporation Law; or



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for any transaction from which the director derived an improper personal benefit.

        Our amended and restated certificate of incorporation further provides that we must indemnify our directors to the fullest extent permitted by Delaware law. In addition, our amended and restated bylaws provide that:

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we are required to indemnify our directors and executive officers to the fullest extent permittednot prohibited by Delaware law or any other applicable law, subject to limited exceptions;



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we may indemnify our other officers, employees and other agents to the extent that we indemnify our officers and directors, unless otherwise prohibited byas set forth in Delaware law our amended and restated certificate of incorporation, our amended and restated bylaws or agreements;any other applicable law;



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we are required to advance expenses to our directors and executive officers as incurred in connection with legal proceedings against them for which they may be indemnified; and



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the rights conferred in the amended and restated bylaws are not exclusive.

        We have entered into indemnification agreements with each of our directors and certain officers. These agreements, among other things, require us to indemnify each director and officer to the fullest extent permitted by Delaware law, including indemnification for expenses such as attorneys' fees,

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judgments, fines and settlement amounts incurred by the director or officer in any action or proceeding, including any action by or in the right of Rigel, arising out of the person's services as a director or officer of us, any subsidiary of ours or any other company or enterprise to which the person provides services at our request. At present, we are not aware of any pending or threatened litigation.litigation or proceeding involving any of our directors, officers, employees or agents in which indemnification would be required or permitted. We believe that our charter provisions and indemnification agreements are necessary to attract and retain qualified persons as directors and officers.

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        The underwriting agreement that we might enter into (Exhibit 1.1) will provide for indemnification by any underwriters of Rigel, our directors, our officers who sign the registration statement and our controlling persons for some liabilities, including liabilities arising under the Securities Act.

Item 16.ITEM 16. EXHIBITSExhibits and Financial Statement Schedules

(a)

Exhibits

*(1)

To be providedfiled by Amendmentamendment or as an exhibit to a current report of the registrant on Form 8-K and incorporated herein by reference.

(2)

Filed as an exhibit to the registrant's current report on Form 8-K (File No. 000-29889), originally filed with the Securities and Exchange Commission on June 24, 2003, and incorporated herein by reference.

(3)

Filed as an exhibit to the registrant's registration statement on Form S-1 or amendments thereto (File No. 333-45864), originally filed with the Securities and Exchange Commission on September 15, 2000, and incorporated herein by reference.

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Item 17.ITEM 17. UNDERTAKINGSUndertakings

(a)

The undersigned registrant hereby undertakes:

        (1)   To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

        (i)    Toto include any prospectus required by Sectionsection 10(a)(3) of the Securities Act of 1933;Act;

        (ii)   Toto reflect in the prospectus any facts or events arising after the effective date of thisthe registration statement (or the most recent post-effective amendment hereof)thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in thisthe registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Securities and Exchange Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than a 20 percent change in the maximum aggregate offering price set forth in the "Calculation of Registration Fee" table in the effective registration statement; and

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        (iii)  Toto include any material information with respect to the plan of distribution not previously disclosed in thisthe registration statement or any material change to such information in thisthe registration statement;statement.

provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply if the registration statement is on Form S-3, Form S-8 or Form F-3, and the information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed with or furnished to the Securities and Exchange Commission by the registrant pursuant to Sectionsection 13 or Section 15(d) of the Securities Exchange Act of 1934 that are incorporated by reference in the registration statement.

        (2)   That, for the purpose of determining any liability under the Securities Act, of 1933, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of the securities at that time shall be deemed to be the initialbona fide offering thereof.

        (3)   To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of this offering.

        (4)   That: (i) for purposes of determining any liability under the Securities Act, the information omitted from the form of prospectus filed as part of the registration statement in reliance upon Rule 430A and contained in the form of prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or 497(h) under the Securities Act shall be deemed to be part of the registration statement as of the time it was declared effective; and (ii) for the purpose of determining any liability under the Securities Act, each post-effective amendment that contains a form of prospectus shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initialbona fide offering thereof.

        (3)   To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(b)

The undersigned registrant hereby undertakes that,        (5)   That, for purposes of determining any liability under the Securities Act, of 1933, each filing of the registrant's annual report pursuant to Sectionsection 13(a) or Sectionsection 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan's annual report pursuant to Sectionsection 15(d) of the Securities Exchange Act of 1934)Act) that is incorporated by reference in thisthe registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initialbona fide offering thereof.

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(c)

        (6)   To file an application for the purpose of determining the eligibility of the trustee to act under subsection (a) of section 310 of the Trust Indenture Act in accordance with the rules and regulations prescribed by the Securities and Exchange Commission under section 305(b)(2) of the Trust Indenture Act.

Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission suchthis form of indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against suchthese liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by sucha director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of suchthis issue.

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SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of South San Francisco, State of California, on May 20, 2003.January 8, 2004.

POWER OF ATTORNEY

        KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints James M. Gower and James H. Welch, and each of them, as true and lawful attorneys-in-fact and agents, with full powers of substitution and resubstitution, for them and in their name, place and stead, in any and all capacities, to sign any and all amendments (including pre-effective and post-effective amendments and registration statements filed pursuant to Rule 462) to this registration statement, and to file the same, with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission (the "SEC"), and generally to do all such things in their names and behalf in their capacities as officers and directors to enable Rigel to comply with the provisions of the Securities Act of 1933 and all requirements of the Securities and Exchange Commission,SEC, granting unto said attorneys-in-fact and agents, and each of them, full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully to all intents and purposes as he or she might or could do in person, ratifying and confirming all that said attorneys-in-fact and agents, or any of them, or their or his or her substitutes or substitute, may lawfully do or cause to be done by virtue hereof.

        Pursuant to the requirements of the Securities Act of 1933, this registration statement has been signed below by the following persons in the capacities and on the dates indicated.

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INDEX TO EXHIBITS

*(1)

To be providedfiled by Amendmentamendment or as an exhibit to a current report of the registrant on Form 8-K and incorporated herein by reference.

(2)

Filed as an exhibit to the registrant's current report on Form 8-K (File No. 000-29889), originally filed with the Securities and Exchange Commission on June 24, 2003, and incorporated herein by reference.

(3)

Filed as an exhibit to the registrant's registration statement on Form S-1 or amendments thereto (File No. 333-45864), originally filed with the Securities and Exchange Commission on September 15, 2000, and incorporated herein by reference.

QuickLinks

ITEMItem 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTIONOther Expenses of Issuance and Distribution
ITEMItem 15. INDEMNIFICATION OF DIRECTORS AND OFFICERSIndemnification of Officers and Directors
ITEMItem 16. EXHIBITSExhibits and Financial Statement Schedules
ITEMItem 17. UNDERTAKINGSUndertakings