Exhibit 1.1


                                5,000,000 SHARES


                           ONYX PHARMACEUTICALS, INC.

                    COMMON STOCK, PAR VALUE $0.001 PER SHARE




                             UNDERWRITING AGREEMENT

July 21, 2003




Morgan Stanley & Co. Incorporated
Lehman Brothers Inc.
SG Cowen Securities Corporation
c/o  Morgan Stanley & Co.
     Incorporated
     1585 Broadway
     New York, New York 10036

Dear Sirs and Mesdames:

      Onyx Pharmaceuticals, Inc., a Delaware corporation (the "COMPANY"),
proposes to issue and sell to the several Underwriters named in Schedule I
hereto (the "UNDERWRITERS") 5,000,000 shares of its common stock, par value
$0.001 per share (the "FIRM SHARES"). The Company also proposes to issue and
sell to the several Underwriters not more than an additional 750,000 shares of
its common stock, par value $0.001 per share (the "ADDITIONAL SHARES"), if and
to the extent that you, as representatives of the Underwriters for the offering,
shall have determined to exercise, on behalf of the Underwriters, the right to
purchase such shares of common stock granted to the Underwriters in Section 2
hereof. The Firm Shares and the Additional Shares are hereinafter collectively
referred to as the "SHARES." The shares of common stock, par value $0.001 per
share, of the Company are hereinafter referred to as the "COMMON STOCK."

      The Company has filed with the Securities and Exchange Commission (the
"COMMISSION") a shelf registration statement on Form S-3 (File No. 333-106070),
including a prospectus, relating to the Shares, and the registration statement
has been declared effective under the Securities Act of 1933, as amended (the
"SECURITIES ACT"). The shelf registration statement, at the time it was declared
effective by the Commission, is hereinafter referred to as the "REGISTRATION
STATEMENT." If the Company has filed an abbreviated registration statement to
register additional shares of Common Stock pursuant to Rule 462(b) under the
Securities Act (the "RULE 462 REGISTRATION STATEMENT"), then any reference
herein to the term "REGISTRATION STATEMENT" shall be deemed to include such Rule
462 Registration Statement. The form of prospectus, as included in the
Registration Statement, is hereinafter referred to as the "BASE PROSPECTUS." The
prospectus supplement relating to the Shares as filed with the Commission
pursuant to and in accordance with Rule 424(b) ("RULE 424(B)") under the
Securities Act is hereinafter referred to as the "PROSPECTUS SUPPLEMENT." The
term "PROSPECTUS" means the Base Prospectus together with the Prospectus
Supplement. The term "PRELIMINARY PROSPECTUS" means a preliminary

prospectus supplement specifically relating to the Shares together with the Base
Prospectus. As used herein, the terms "BASE PROSPECTUS," "PROSPECTUS,"
"REGISTRATION STATEMENT," "RULE 462 REGISTRATION STATEMENT," "PRELIMINARY
PROSPECTUS" and "PROSPECTUS SUPPLEMENT" shall include in each case the
documents, if any, incorporated by reference therein. The terms "SUPPLEMENT" and
"AMENDMENT" or "AMEND" as used in this Agreement with respect to the
Registration Statement or the Prospectus shall include all documents filed
subsequent to the date of the Base Prospectus by the Company with the Commission
pursuant to the Securities Exchange Act of 1934, as amended (the "EXCHANGE
ACT"), that are deemed to be incorporated by reference in the Prospectus.

      1.    Representations and Warranties. The Company represents and warrants
to and agrees with each of the Underwriters that:

            (a) The Registration Statement has become effective; no stop order
      suspending the effectiveness of the Registration Statement is in effect,
      and no proceedings for such purpose are pending before or threatened by
      the Commission.

            (b) (i) Each document, if any, filed or to be filed pursuant to the
      Exchange Act and incorporated by reference in the Prospectus complied or
      will comply when so filed in all material respects with the Exchange Act
      and the applicable rules and regulations of the Commission thereunder,
      (ii) the Registration Statement, when it became effective, did not contain
      and, as amended or supplemented, if applicable, will not contain any
      untrue statement of a material fact or omit to state a material fact
      required to be stated therein or necessary to make the statements therein
      not misleading, (iii) the Registration Statement and the Prospectus comply
      and, as amended or supplemented, if applicable, will comply in all
      material respects with the Securities Act and the applicable rules and
      regulations of the Commission thereunder and (iv) the Prospectus does not
      contain and, as amended or supplemented, if applicable, will not contain
      any untrue statement of a material fact or omit to state a material fact
      necessary to make the statements therein, in the light of the
      circumstances under which they were made, not misleading, except that the
      representations and warranties set forth in this paragraph do not apply to
      statements or omissions in the Registration Statement or the Prospectus
      based upon information relating to any Underwriter furnished to the
      Company in writing by such Underwriter through you expressly for use
      therein.

            (c) The Company has been duly incorporated, is validly existing as a
      corporation in good standing under the laws of the jurisdiction of its
      incorporation, has the corporate power and authority to own its property
      and to conduct its business as described in the Prospectus and is duly
      qualified to transact business and is in good standing in each
      jurisdiction


                                       2

      in which the conduct of its business or its ownership or leasing of
      property requires such qualification, except to the extent that the
      failure to be so qualified or be in good standing would not have a
      material adverse effect on the Company.

            (d) The Company has no subsidiaries.

            (e) This Agreement has been duly authorized, executed and delivered
      by the Company.

            (f) The authorized capital stock of the Company conforms as to legal
      matters to the description thereof contained in the Prospectus.

            (g) The shares of Common Stock of the Company outstanding prior to
      the issuance of the Shares have been duly authorized and are validly
      issued, fully paid and non-assessable.

            (h) The Shares have been duly authorized and, when issued and
      delivered in accordance with the terms of this Agreement, will be validly
      issued, fully paid and non-assessable, and the issuance of such Shares
      will not be subject to any preemptive or similar rights.

            (i) The execution and delivery by the Company of, and the
      performance by the Company of its obligations under, this Agreement will
      not contravene any provision of applicable law or the certificate of
      incorporation or by-laws of the Company or any agreement or other
      instrument binding upon the Company that is material to the Company or any
      judgment, order or decree of any governmental body, agency or court having
      jurisdiction over the Company, and no consent, approval, authorization or
      order of, or qualification with, any governmental body or agency is
      required for the performance by the Company of its obligations under this
      Agreement, except such as may be required by the securities or Blue Sky
      laws of the various states in connection with the offer and sale of the
      Shares.

            (j) There has not occurred any material adverse change, or any
      development involving a prospective material adverse change, in the
      condition, financial or otherwise, or in the earnings, business or
      operations of the Company, from that set forth in the Prospectus
      (exclusive of any amendments or supplements thereto subsequent to the date
      of this Agreement).

            (k) There are no legal or governmental proceedings pending or
      threatened to which the Company is a party or to which any of the
      properties of the Company is subject that are required to be described in
      the Registration Statement or the Prospectus and are not so described or


                                       3

      any statutes, regulations, contracts or other documents that are required
      to be described in the Registration Statement or the Prospectus or to be
      filed as exhibits to the Registration Statement that are not described or
      filed as required.

            (l) Each preliminary prospectus filed as part of the registration
      statement as originally filed or as part of any amendment thereto, or
      filed pursuant to Rule 424 under the Securities Act, complied when so
      filed in all material respects with the Securities Act and the applicable
      rules and regulations of the Commission thereunder.

            (m) The Company is not, and after giving effect to the offering and
      sale of the Shares and the application of the proceeds thereof as
      described in the Prospectus will not be, required to register as an
      "investment company" as such term is defined in the Investment Company Act
      of 1940, as amended.

            (n) The Company (i) is in compliance with any and all applicable
      foreign, federal, state and local laws and regulations relating to the
      protection of human health and safety, the environment or hazardous or
      toxic substances or wastes, pollutants or contaminants ("ENVIRONMENTAL
      LAWS"), (ii) has received all permits, licenses or other approvals
      required of it under applicable Environmental Laws to conduct its business
      and (iii) is in compliance with all terms and conditions of any such
      permit, license or approval, except where such noncompliance with
      Environmental Laws, failure to receive required permits, licenses or other
      approvals or failure to comply with the terms and conditions of such
      permits, licenses or approvals would not, singly or in the aggregate, have
      a material adverse effect on the Company.

            (o) To the best of the Company's knowledge, there are no costs or
      liabilities associated with Environmental Laws (including, without
      limitation, any capital or operating expenditures required for clean-up,
      closure of properties or compliance with Environmental Laws or any permit,
      license or approval, any related constraints on operating activities and
      any potential liabilities to third parties) which would, singly or in the
      aggregate, have a material adverse effect on the Company.

            (p) There are no contracts, agreements or understandings between the
      Company and any person granting such person the right to require the
      Company to file a registration statement under the Securities Act with
      respect to any securities of the Company or to require the Company to
      include such securities with the Shares registered pursuant to the
      Registration Statement other than such contracts, agreements or
      understandings as have been waived in connection with the transactions
      contemplated hereby.


                                       4

            (q) The Company owns or possesses adequate licenses or other rights
      to use all material patents, technology, trademarks, trade names,
      know-how, copyrights and other intellectual property (collectively,
      "INTELLECTUAL PROPERTY RIGHTS") currently employed or planned to be
      employed by it in the conduct of its business as described in the
      Prospectus, and the Company has not received any notice of infringement of
      or conflict with asserted rights of others with respect to any
      intellectual property rights which if the subject of a decision, ruling,
      finding or settlement would reasonably be expected to result in a material
      adverse effect on the general affairs, management, financial position,
      stockholders' equity, results of operations, business or prospects of the
      Company; and the discoveries, inventions, products, product candidates or
      processes of the Company referred to in the Prospectus do not, to the
      actual knowledge of the Company without further investigation, infringe
      any intellectual property right of any third party, or any discovery,
      invention, product or process that, to the actual knowledge of the Company
      without further investigation, is the object of a patent application filed
      by any third party, except for any such infringement which would not have
      a material adverse effect on the general affairs, management, financial
      position, stockholders' equity, results of operations, business or
      prospects of the Company.

            (r) The statistical and market-related data included in the
      Prospectus are based on or derived from sources which the Company believes
      to be reliable and accurate.

            (s) The Company has established and maintains disclosure controls
      and procedures (as such term is defined in Rule 13a-14 under the Exchange
      Act), which (i) are designed to ensure that material information relating
      to the Company is made known to the Company's principal executive officer
      and its principal financial officer by others within those entities,
      particularly during the periods in which the periodic reports required
      under the Exchange Act are being prepared; (ii) provide for the periodic
      evaluation of the effectiveness of such disclosure controls and procedures
      as of a date within 90 days prior to the filing of the Company's most
      recent annual or quarterly report with the Commission; and (iii) are
      effective in all material respects to perform the functions for which they
      were established.

            (t) Based on the evaluation of its disclosure controls and
      procedures, the Company is not aware of (i) any significant deficiency in
      the design or operation of internal controls which could adversely affect
      the Company's ability to record, process, summarize and report financial
      data or any material weaknesses in internal controls; or (ii) any fraud,
      whether or not material, that involves management or other employees who
      have a significant role in the Company's internal controls.


                                       5

            (u) Since the date of the most recent evaluation of such disclosure
      controls and procedures, there have been no changes that have materially
      affected, or are reasonably likely to materially affect, the Company's
      internal control over financial reporting, including any corrective
      actions with regard to significant deficiencies and material weaknesses.

            (v) Except as described in the Prospectus, there are no material
      off-balance sheet arrangements (as defined in Item 303 of Regulation S-K)
      that have or are reasonably likely to have a material current or future
      effect on the Company's financial condition, revenues or expenses, changes
      in financial condition, results of operations, liquidity, capital
      expenditures or capital resources.

            (w) Except as described in the Prospectus and as preapproved in
      accordance with the requirements set forth in Section 10A of the Exchange
      Act, Ernst & Young LLP has not been engaged by the Company to perform any
      "prohibited activities" (as defined in Section 10A of the Exchange Act).

            (x) The Company's Board of Directors has validly appointed an audit
      committee whose composition satisfies the requirements of Rule 4350(d)(2)
      of the Rules of the National Association of Securities Dealers, Inc. (the
      "NASD RULES") and the Board of Directors and/or the audit committee has
      adopted a charter that satisfies the requirements of Rule 4350(d)(1) of
      the NASD Rules. The audit committee has reviewed the adequacy of its
      charter within the past twelve months.

      2.    Agreements to Sell and Purchase. The Company hereby agrees to sell
to the several Underwriters, and each Underwriter, upon the basis of the
representations and warranties herein contained, but subject to the conditions
hereinafter stated, agrees, severally and not jointly, to purchase from the
Company the respective numbers of Firm Shares set forth in Schedule I hereto
opposite its name at $14.335 a share (the "PURCHASE PRICE").

      On the basis of the representations and warranties contained in this
Agreement, and subject to its terms and conditions, the Company agrees to sell
to the Underwriters the Additional Shares, and the Underwriters shall have the
right to purchase, severally and not jointly, up to 750,000 Additional Shares at
the Purchase Price. You may exercise this right on behalf of the Underwriters in
whole or from time to time in part by giving written notice of each election to
exercise the option not later than 30 days after the date of this Agreement. Any
exercise notice shall specify the number of Additional Shares to be purchased by
the Underwriters and the date on which such shares are to be purchased. Each
purchase date must be at least one business day after the written notice is
given and may not be earlier than the closing date for the Firm Shares nor later
than ten


                                       6

business days after the date of such notice. Additional Shares may be purchased
as provided in Section 4 hereof solely for the purpose of covering
over-allotments made in connection with the offering of the Firm Shares. On each
day, if any, that Additional Shares are to be purchased (an "OPTION CLOSING
DATE"), each Underwriter agrees, severally and not jointly, to purchase the
number of Additional Shares (subject to such adjustments to eliminate fractional
shares as you may determine) that bears the same proportion to the total number
of Additional Shares to be purchased on such Option Closing Date as the number
of Firm Shares set forth in Schedule I hereto opposite the name of such
Underwriter bears to the total number of Firm Shares.

      The Company hereby agrees that, without the prior written consent of
Morgan Stanley & Co. Incorporated on behalf of the Underwriters, it will not,
during the period ending 90 days after the date of the Prospectus Supplement,
(i) offer, pledge, sell, contract to sell, sell any option or contract to
purchase, purchase any option or contract to sell, grant any option, right or
warrant to purchase, lend, or otherwise transfer or dispose of, directly or
indirectly, any shares of Common Stock or any securities convertible into or
exercisable or exchangeable for Common Stock or (ii) enter into any swap or
other arrangement that transfers to another, in whole or in part, any of the
economic consequences of ownership of the Common Stock, whether any such
transaction described in clause (i) or (ii) above is to be settled by delivery
of Common Stock or such other securities, in cash or otherwise. The foregoing
sentence shall not apply to (A) the Shares to be sold hereunder, (B) the
issuance by the Company of shares of Common Stock upon the exercise of an option
or warrant or the conversion of a security outstanding on the date hereof of
which the Underwriters have been advised in writing, (C) the grant of options to
purchase Common Stock under the Company's stock option plans existing on the
date hereof, or (D) the issuance by the Company of shares of Common Stock under
the Company's employee stock purchase plan existing on the date hereof.

      3.    Terms of Public Offering. The Company is advised by you that the
Underwriters propose to make a public offering of their respective portions of
the Shares as soon after the Registration Statement and this Agreement have
become effective as in your judgment is advisable. The Company is further
advised by you that the Shares are to be offered to the public initially at
$15.25 a share (the "PUBLIC OFFERING PRICE") and to certain dealers selected by
you at a price that represents a concession not in excess of $0.59 a share under
the Public Offering Price.

      4.    Payment and Delivery. Payment for the Firm Shares shall be made to
the Company in Federal or other funds immediately available in New York City
against delivery of such Firm Shares for the respective accounts of the several
Underwriters at 10:00 a.m., New York City time, on July 25, 2003, or at such
other time on the same or such other date, not later than August 1, 2003, as


                                       7

shall be designated in writing by you. The time and date of such payment are
hereinafter referred to as the "CLOSING DATE."

      Payment for any Additional Shares shall be made to the Company in Federal
or other funds immediately available in New York City against delivery of such
Additional Shares for the respective accounts of the several Underwriters at
10:00 a.m., New York City time, on the date specified in the corresponding
notice described in Section 2 or at such other time on the same or on such other
date, in any event not later than September 3, 2003, as shall be designated in
writing by you. The time and date of such payment are hereinafter referred to as
the "OPTION CLOSING DATE."

      The Firm Shares and Additional Shares shall be registered in such names
and in such denominations as you shall request in writing not later than one
full business day prior to the Closing Date or the applicable Option Closing
Date, as the case may be. The Firm Shares and Additional Shares shall be
delivered to you on the Closing Date or an Option Closing Date, as the case may
be, for the respective accounts of the several Underwriters, with any transfer
taxes payable in connection with the transfer of the Shares to the Underwriters
duly paid, against payment of the Purchase Price therefor.

      5.    Conditions to the Underwriters' Obligations. The obligations of the
Company to sell the Shares to the Underwriters and the several obligations of
the Underwriters to purchase and pay for the Shares on the Closing Date are
subject to the condition that the Registration Statement shall have become
effective not later than 2:00 p.m. (New York City time) on the date hereof.

      The several obligations of the Underwriters are subject to the following
further conditions:

            (a)   Subsequent to the execution and delivery of this Agreement and
      prior to the Closing Date:

                  (i)   there shall not have occurred any downgrading, nor shall
            any notice have been given of any intended or potential downgrading
            or of any review for a possible change that does not indicate the
            direction of the possible change, in the rating accorded any of the
            Company's securities by any "nationally recognized statistical
            rating organization," as such term is defined for purposes of Rule
            436(g)(2) under the Securities Act; and

                  (ii)  there shall not have occurred any change, or any
            development involving a prospective change, in the condition,
            financial or otherwise, or in the earnings, business or operations
            of the Company from that set forth in the Prospectus (exclusive of
            any amendments or supplements thereto subsequent to the date of


                                       8

            this Agreement) that, in your judgment, is material and adverse and
            that makes it, in your judgment, impracticable to market the Shares
            on the terms and in the manner contemplated in the Prospectus.

            (b)   The Underwriters shall have received on the Closing Date a
      certificate, dated the Closing Date and signed by an executive officer of
      the Company, to the effect set forth in Section 5(a)(i) above and to the
      effect that the representations and warranties of the Company contained in
      this Agreement are true and correct as of the Closing Date and that the
      Company has complied with all of the agreements and satisfied all of the
      conditions on its part to be performed or satisfied hereunder on or before
      the Closing Date. The officer signing and delivering such certificate may
      rely upon the best of his or her knowledge as to proceedings threatened.

            (c)   The Underwriters shall have received on the Closing Date an
      opinion of Cooley Godward LLP, outside counsel for the Company, dated the
      Closing Date, to the effect that:

                  (i)   the Company has been duly incorporated, is validly
            existing as a corporation in good standing under the laws of the
            jurisdiction of its incorporation, has the corporate power and
            authority to own its property and to conduct its business as
            described in the Prospectus and is duly qualified to transact
            business and, to such counsel's knowledge, is in good standing in
            each jurisdiction in which the conduct of its business or its
            ownership or leasing of property requires such qualification, except
            to the extent that the failure to be so qualified or be in good
            standing would not have a material adverse effect on the Company;

                  (ii)  the authorized capital stock of the Company conforms as
            to legal matters to the description thereof contained in the
            Prospectus under the caption "Description of Capital Stock";

                  (iii) the shares of Common Stock outstanding prior to the
            issuance of the Shares have been duly authorized and are validly
            issued, fully paid and non-assessable;

                  (iv)  the Shares have been duly authorized and, when issued
            and delivered in accordance with the terms of this Agreement, will
            be validly issued, fully paid and non-assessable, and the issuance
            of such Shares will not be subject to any preemptive or, to such
            counsel's knowledge, similar rights;

                  (v)   this Agreement has been duly authorized, executed and
            delivered by the Company;


                                       9

                  (vi)   the execution and delivery by the Company of, and the
            performance by the Company of its obligations under, this Agreement
            will not contravene any provision of applicable law or the
            certificate of incorporation or by-laws of the Company or, to the
            best of such counsel's knowledge, any agreement or other instrument
            binding upon the Company that is filed as an exhibit to the
            Registration Statement or any document incorporated by reference
            into the Registration Statement as of the Closing Date or, to the
            best of such counsel's knowledge, any judgment, order or decree of
            any governmental body, agency or court having jurisdiction over the
            Company, and no consent, approval, authorization or order of, or
            qualification with, any governmental body or agency is required for
            the performance by the Company of its obligations under this
            Agreement, except as have been made or obtained or except as such as
            may be required by the securities or Blue Sky laws of the various
            states in connection with the offer and sale of the Shares;

                  (vii)  the statements in (A) the Base Prospectus under the
            captions "Description of Capital Stock" and "Plan of Distribution,"
            (B) the Prospectus Supplement under the caption "Underwriters" and
            (C) the Registration Statement in Item 15, in each case insofar as
            such statements constitute matters of law, summaries of legal
            matters, provisions of the Company's certificate of incorporation or
            by-laws, or legal proceedings, or legal conclusions, have been
            reviewed by such counsel and fairly summarize in all material
            respects the matters referred to therein;

                  (viii) after due inquiry, such counsel does not know of any
            legal or governmental proceedings pending or overtly threatened to
            which the Company is a party or to which any of the properties of
            the Company is subject that are required to be described in the
            Registration Statement or the Prospectus and are not so described or
            of any statutes, regulations, contracts or other documents that are
            required to be described in the Registration Statement or the
            Prospectus or to be filed as exhibits to the Registration Statement
            that are not described or filed as required;

                  (ix)   the Company is not, and after giving effect to the
            offering and sale of the Shares and the application of the proceeds
            thereof as described in the Prospectus will not be, required to
            register as an "investment company" as such term is defined in the
            Investment Company Act of 1940, as amended; and

                  (x)    to such counsel's knowledge, (A) each document filed
            pursuant to the Exchange Act and incorporated by reference


                                       10

            in the Registration Statement and the Prospectus (except for the
            financial statements and financial schedules and other financial and
            statistical data included therein, as to which such counsel need not
            express any belief) complied as to form when so filed in all
            material respects with the requirements of the Exchange Act and the
            applicable rules and regulations of the Commission thereunder; and
            (B) the Registration Statement and the Prospectus (except for the
            financial statements and financial schedules and other financial and
            statistical data included therein, as to which such counsel need not
            express any belief) comply as to form in all material respects with
            the requirements of the Securities Act and the applicable rules and
            regulations of the Commission thereunder.

            In addition, counsel shall confirm that in connection with the
      preparation of the Registration Statement and the Prospectus Supplement,
      such counsel has participated in conferences with officers and other
      representatives of the Company, representatives of the independent public
      accountants of the Company and representatives of the Underwriters at
      which the contents of the Registration Statement and Prospectus were
      discussed and, although such counsel is not passing upon and does not
      assume responsibility for the accuracy, completeness or fairness of the
      statements contained in the Registration Statement or Prospectus (except
      as and to the extent stated in subparagraphs (ii) and (vii) above), on the
      basis of the foregoing, nothing has come to such counsel's attention that
      causes such counsel to believe that (A) the Registration Statement or the
      Base Prospectus (except for the financial statements and financial
      schedules and other financial and statistical data included therein, as to
      which such counsel need not express any belief) at the time the
      Registration Statement became effective contained an untrue statement of a
      material fact or omitted to state a material fact required to be stated
      therein or necessary to make the statements therein not misleading or (B)
      the Prospectus (except for the financial statements and financial
      schedules and other financial and statistical data included therein, as to
      which such counsel need not express any belief) as of its date or as of
      the Closing Date contained or contains an untrue statement of a material
      fact or omitted or omits to state a material fact necessary in order to
      make the statements therein, in the light of the circumstances under which
      they were made, not misleading.

            (d)   The Underwriters shall have received on the Closing Date an
      opinion of Latham & Watkins LLP, counsel for the Underwriters, dated the
      Closing Date, covering the matters referred to in Sections 5(c)(iv),
      5(c)(v), 5(c)(vii) (but only as to the statements in the Prospectus under
      "Underwriters") and a confirmation to the effect set forth in the last
      paragraph of Section 5(c) above.


                                       11

            With respect to the confirmation set forth in last paragraph of
      Section 5(c) above, Cooley Godward LLP and Latham & Watkins LLP may state
      that their beliefs are based upon their participation in the preparation
      of the Registration Statement and Prospectus and any amendments or
      supplements thereto, and review and discussion of the contents thereof
      (including the documents incorporated by reference), but are without
      independent check or verification, except as specified.

            The opinion of Cooley Godward LLP described in Section 5(c) above
      shall be rendered to the Underwriters at the request of the Company and
      shall so state therein.

            (e)   The Underwriters shall have received, on each of the date
      hereof and the Closing Date, a letter dated the date hereof or the Closing
      Date, as the case may be, in form and substance satisfactory to the
      Underwriters, from Ernst & Young LLP, independent public accountants,
      containing statements and information of the type ordinarily included in
      accountants' "comfort letters" to underwriters with respect to the
      financial statements and certain financial information contained in or
      incorporated by reference into the Registration Statement and the
      Prospectus; provided that the letter delivered on the Closing Date shall
      use a "cut-off date" not earlier than the date hereof.

            (f)   The "lock-up" agreements, each substantially in the form of
      Exhibit A hereto (except for the lock-up agreement to be executed by D.
      Scott Geyer, Senior Vice President, Product Development, of the Company,
      which shall be in the form agreed upon by the Company and the
      Underwriters) and , between you and the officers and directors of the
      Company relating to sales and certain other dispositions of shares of
      Common Stock or certain other securities, delivered to you on or before
      the date hereof, shall be in full force and effect on the Closing Date.

      The several obligations of the Underwriters to purchase Additional Shares
hereunder are subject to the delivery to you on the applicable Option Closing
Date of such documents as you may reasonably request with respect to the good
standing of the Company, the due authorization and issuance of the Additional
Shares to be sold on such Option Closing Date and other matters related to the
issuance of such Additional Shares.

      6.    Covenants of the Company. In further consideration of the agreements
of the Underwriters herein contained, the Company covenants with each
Underwriter as follows:

            (a)   To furnish to you, without charge, four photocopies of the
      signed Registration Statement (including exhibits thereto and documents
      incorporated by reference) and for delivery to each other Underwriter a


                                       12

      conformed copy of the Registration Statement (without exhibits thereto but
      including documents incorporated by reference) and to furnish to you in
      New York City, without charge, prior to 10:00 a.m. New York City time on
      the business day next succeeding the date of this Agreement and during the
      period mentioned in Section 6(c) below, as many copies of the Prospectus,
      any documents incorporated by reference, and any supplements and
      amendments thereto or to the Registration Statement as you may reasonably
      request.

            (b)   Before amending or supplementing the Registration Statement or
      the Prospectus, to furnish to you a copy of each such proposed amendment
      or supplement and not to file any such proposed amendment or supplement to
      which you reasonably object, and to file with the Commission within the
      applicable period specified in Rule 424(b) under the Securities Act any
      prospectus required to be filed pursuant to such Rule.

            (c)   If, during such period after the first date of the public
      offering of the Shares as in the opinion of counsel for the Underwriters
      the Prospectus is required by law to be delivered in connection with sales
      by an Underwriter or dealer, any event shall occur or condition exist as a
      result of which it is necessary to amend or supplement the Prospectus in
      order to make the statements therein, in the light of the circumstances
      when the Prospectus is delivered to a purchaser, not misleading, or if, in
      the opinion of counsel for the Underwriters, it is necessary to amend or
      supplement the Prospectus to comply with applicable law, forthwith to
      prepare, file with the Commission and furnish, at its own expense, to the
      Underwriters and to the dealers (whose names and addresses you will
      furnish to the Company) to which Shares may have been sold by you on
      behalf of the Underwriters and to any other dealers upon request, either
      amendments or supplements to the Prospectus so that the statements in the
      Prospectus as so amended or supplemented will not, in the light of the
      circumstances when the Prospectus is delivered to a purchaser, be
      misleading or so that the Prospectus, as amended or supplemented, will
      comply with law.

            (d)   To endeavor to qualify the Shares for offer and sale under the
      securities or Blue Sky laws of such jurisdictions as you shall reasonably
      request.

            (e)   To make generally available to the Company's security holders
      and to you as soon as practicable an earning statement covering the
      twelve-month period ending September 30, 2004 that satisfies the
      provisions of Section 11(a) of the Securities Act and the rules and
      regulations of the Commission thereunder.


                                       13

            (f)   Whether or not the transactions contemplated in this Agreement
      are consummated or this Agreement is terminated, to pay or cause to be
      paid all expenses incident to the performance of its obligations under
      this Agreement, including: (i) the fees, disbursements and expenses of the
      Company's counsel and the Company's accountants in connection with the
      registration and delivery of the Shares under the Securities Act and all
      other fees or expenses in connection with the preparation and filing of
      the Registration Statement, any preliminary prospectus, the Prospectus and
      amendments and supplements to any of the foregoing, including all printing
      costs associated therewith, and the mailing and delivering of copies
      thereof to the Underwriters and dealers, in the quantities hereinabove
      specified, (ii) all costs and expenses related to the transfer and
      delivery of the Shares to the Underwriters, including any transfer or
      other taxes payable thereon, (iii) the cost of printing or producing any
      Blue Sky or Legal Investment memorandum in connection with the offer and
      sale of the Shares under state securities laws and all expenses in
      connection with the qualification of the Shares for offer and sale under
      state securities laws as provided in Section 6(d) hereof, including filing
      fees and the reasonable fees and disbursements of counsel for the
      Underwriters in connection with such qualification and in connection with
      the Blue Sky or Legal Investment memorandum, (iv) all filing fees and the
      reasonable fees and disbursements of counsel to the Underwriters incurred
      in connection with the review and qualification of the offering of the
      Shares by the National Association of Securities Dealers, Inc., (v) all
      costs and expenses incident to listing the Shares on the Nasdaq National
      Market, (vi) the cost of printing certificates representing the Shares,
      (vii) the costs and charges of any transfer agent, registrar or
      depositary, (viii) the costs and expenses of the Company relating to
      investor presentations on any "road show" undertaken in connection with
      the marketing of the offering of the Shares, including, without
      limitation, expenses associated with the production of road show slides
      and graphics, fees and expenses of any consultants engaged in connection
      with the road show presentations with the prior approval of the Company,
      travel and lodging expenses of the representatives and officers of the
      Company and any such consultants, and the cost of any aircraft chartered
      in connection with the road show, (ix) the document production charges and
      expenses associated with printing this Agreement and (x) all other costs
      and expenses incident to the performance of the obligations of the Company
      hereunder for which provision is not otherwise made in this Section. It is
      understood, however, that except as provided in this Section, Section 7
      entitled "Indemnity and Contribution", and the last paragraph of Section 9
      below, the Underwriters will pay all of their costs and expenses,
      including fees and disbursements of their counsel, stock transfer taxes
      payable on resale of any of the Shares by them and any advertising
      expenses connected with any offers they may make.


                                       14

      7.    Indemnity and Contribution.

            (a)   The Company agrees to indemnify and hold harmless each
      Underwriter, each person, if any, who controls any Underwriter within the
      meaning of either Section 15 of the Securities Act or Section 20 of the
      Exchange Act, and each affiliate of any Underwriter within the meaning of
      Rule 405 under the Securities Act, from and against any and all losses,
      claims, damages and liabilities (including, without limitation, any legal
      or other expenses reasonably incurred in connection with defending or
      investigating any such action or claim) caused by any untrue statement or
      alleged untrue statement of a material fact contained in the Registration
      Statement or any amendment thereof, any preliminary prospectus or the
      Prospectus (as amended or supplemented if the Company shall have furnished
      any amendments or supplements thereto), or caused by any omission or
      alleged omission to state therein a material fact required to be stated
      therein or necessary to make the statements therein not misleading, except
      insofar as such losses, claims, damages or liabilities are caused by any
      such untrue statement or omission or alleged untrue statement or omission
      based upon information relating to any Underwriter furnished to the
      Company in writing by such Underwriter through you expressly for use
      therein; provided, however, that the foregoing indemnity agreement with
      respect to any preliminary prospectus shall not inure to the benefit of
      any Underwriter or affiliate of an Underwriter from whom the person
      asserting any such losses, claims, damages or liabilities purchased
      Shares, or any person controlling such Underwriter, if a copy of the
      Prospectus (as then amended or supplemented if the Company shall have
      furnished any amendments or supplements thereto) was not sent or given by
      or on behalf of such Underwriter to such person, if required by law so to
      have been delivered, at or prior to the written confirmation of the sale
      of the Shares to such person, and if the Prospectus (as so amended or
      supplemented) would have cured the defect giving rise to such losses,
      claims, damages or liabilities, unless such failure is the result of
      noncompliance by the Company with Section 6(a) hereof.

            (b)   Each Underwriter agrees, severally and not jointly, to
      indemnify and hold harmless the Company, its directors, its officers who
      sign the Registration Statement and each person, if any, who controls the
      Company within the meaning of either Section 15 of the Securities Act or
      Section 20 of the Exchange Act to the same extent as the foregoing
      indemnity from the Company to such Underwriter, but only with reference to
      information relating to such Underwriter furnished to the Company in
      writing by such Underwriter through you expressly for use in the
      Registration Statement, any preliminary prospectus, the Prospectus or any
      amendments or supplements thereto.


                                       15

            (c)   In case any proceeding (including any governmental
      investigation) shall be instituted involving any person in respect of
      which indemnity may be sought pursuant to Section 7(a) or 7(b), such
      person (the "INDEMNIFIED PARTY") shall promptly notify the person against
      whom such indemnity may be sought (the "INDEMNIFYING PARTY") in writing
      and the indemnifying party, upon request of the indemnified party, shall
      retain counsel reasonably satisfactory to the indemnified party to
      represent the indemnified party and any others the indemnifying party may
      designate in such proceeding and shall pay the fees and disbursements of
      such counsel related to such proceeding. In any such proceeding, any
      indemnified party shall have the right to retain its own counsel, but the
      fees and expenses of such counsel shall be at the expense of such
      indemnified party unless (i) the indemnifying party and the indemnified
      party shall have mutually agreed to the retention of such counsel or (ii)
      the named parties to any such proceeding (including any impleaded parties)
      include both the indemnifying party and the indemnified party and
      representation of both parties by the same counsel would be inappropriate
      due to actual or potential differing interests between them. It is
      understood that the indemnifying party shall not, in respect of the legal
      expenses of any indemnified party in connection with any proceeding or
      related proceedings in the same jurisdiction, be liable for the fees and
      expenses of more than one separate firm (in addition to any local counsel)
      for all such indemnified parties and that all such fees and expenses shall
      be reimbursed as they are incurred. Such firm shall be designated in
      writing by Morgan Stanley & Co. Incorporated, in the case of parties
      indemnified pursuant to Section 7(a), and by the Company, in the case of
      parties indemnified pursuant to Section 7(b). The indemnifying party shall
      not be liable for any settlement of any proceeding effected without its
      written consent, but if settled with such consent or if there be a final
      judgment for the plaintiff, the indemnifying party agrees to indemnify the
      indemnified party from and against any loss or liability by reason of such
      settlement or judgment. No indemnifying party shall, without the prior
      written consent of the indemnified party, effect any settlement of any
      pending or threatened proceeding in respect of which any indemnified party
      is or could have been a party and indemnity could have been sought
      hereunder by such indemnified party, unless such settlement includes an
      unconditional release of such indemnified party from all liability on
      claims that are the subject matter of such proceeding.

            (d)   To the extent the indemnification provided for in Section 7(a)
      or 7(b) is unavailable to an indemnified party or insufficient in respect
      of any losses, claims, damages or liabilities referred to therein, then
      each indemnifying party under such paragraph, in lieu of indemnifying such
      indemnified party thereunder, shall contribute to the amount paid or
      payable by such indemnified party as a result of such losses, claims,
      damages or liabilities (i) in such proportion as is appropriate to reflect
      the


                                       16

      relative benefits received by the Company on the one hand and the
      Underwriters on the other hand from the offering of the Shares or (ii) if
      the allocation provided by clause 7(d)(i) above is not permitted by
      applicable law, in such proportion as is appropriate to reflect not only
      the relative benefits referred to in clause 7(d)(i) above but also the
      relative fault of the Company on the one hand and of the Underwriters on
      the other hand in connection with the statements or omissions that
      resulted in such losses, claims, damages or liabilities, as well as any
      other relevant equitable considerations. The relative benefits received by
      the Company on the one hand and the Underwriters on the other hand in
      connection with the offering of the Shares shall be deemed to be in the
      same respective proportions as the net proceeds from the offering of the
      Shares (before deducting expenses) received by the Company and the total
      underwriting discounts and commissions received by the Underwriters, in
      each case as set forth in the table on the cover of the Prospectus, bear
      to the aggregate Public Offering Price of the Shares. The relative fault
      of the Company on the one hand and the Underwriters on the other hand
      shall be determined by reference to, among other things, whether the
      untrue or alleged untrue statement of a material fact or the omission or
      alleged omission to state a material fact relates to information supplied
      by the Company or by the Underwriters and the parties' relative intent,
      knowledge, access to information and opportunity to correct or prevent
      such statement or omission. The Underwriters' respective obligations to
      contribute pursuant to this Section 7 are several in proportion to the
      respective number of Shares they have purchased hereunder, and not joint.

            (e)   The Company and the Underwriters agree that it would not be
      just or equitable if contribution pursuant to this Section 7 were
      determined by pro rata allocation (even if the Underwriters were treated
      as one entity for such purpose) or by any other method of allocation that
      does not take account of the equitable considerations referred to in
      Section 7(d). The amount paid or payable by an indemnified party as a
      result of the losses, claims, damages and liabilities referred to in the
      immediately preceding paragraph shall be deemed to include, subject to the
      limitations set forth above, any legal or other expenses reasonably
      incurred by such indemnified party in connection with investigating or
      defending any such action or claim. Notwithstanding the provisions of this
      Section 7, no Underwriter shall be required to contribute any amount in
      excess of the amount by which the total price at which the Shares
      underwritten by it and distributed to the public were offered to the
      public exceeds the amount of any damages that such Underwriter has
      otherwise been required to pay by reason of such untrue or alleged untrue
      statement or omission or alleged omission. No person guilty of fraudulent
      misrepresentation (within the meaning of Section 11(f) of the Securities
      Act) shall be entitled to contribution from any person who was not guilty
      of such fraudulent misrepresentation. The remedies provided for in this
      Section 7 are not


                                       17

      exclusive and shall not limit any rights or remedies which may otherwise
      be available to any indemnified party at law or in equity.

            (f)   The indemnity and contribution provisions contained in this
      Section 7 and the representations, warranties and other statements of the
      Company contained in this Agreement shall remain operative and in full
      force and effect regardless of (i) any termination of this Agreement, (ii)
      any investigation made by or on behalf of any Underwriter, any person
      controlling any Underwriter or any affiliate of any Underwriter or by or
      on behalf of the Company, its officers or directors or any person
      controlling the Company and (iii) acceptance of and payment for any of the
      Shares.

      8.    Termination. The Underwriters may terminate this Agreement by notice
given by you to the Company, if after the execution and delivery of this
Agreement and prior to the Closing Date (i) trading generally shall have been
suspended or materially limited on, or by, as the case may be, any of the New
York Stock Exchange, the American Stock Exchange, the Nasdaq National Market,
the Chicago Board of Options Exchange, the Chicago Mercantile Exchange or the
Chicago Board of Trade, (ii) trading of any securities of the Company shall have
been suspended on any exchange or in any over-the-counter market, (iii) a
material disruption in securities settlement, payment or clearance services in
the United States shall have occurred, (iv) any moratorium on commercial banking
activities shall have been declared by Federal or New York State authorities or
(v) there shall have occurred any outbreak or escalation of hostilities, or any
change in financial markets or any calamity or crisis that, in your judgment, is
material and adverse and which, singly or together with any other event
specified in this clause (v), makes it, in your judgment, impracticable or
inadvisable to proceed with the offer, sale or delivery of the Shares on the
terms and in the manner contemplated in the Prospectus.

      9.    Effectiveness; Defaulting Underwriters. This Agreement shall become
effective upon the execution and delivery hereof by the parties hereto.

      If, on the Closing Date or an Option Closing Date, as the case may be, any
one or more of the Underwriters shall fail or refuse to purchase Shares that it
has or they have agreed to purchase hereunder on such date, and the aggregate
number of Shares which such defaulting Underwriter or Underwriters agreed but
failed or refused to purchase is not more than one-tenth of the aggregate number
of the Shares to be purchased on such date, the other Underwriters shall be
obligated severally in the proportions that the number of Firm Shares set forth
opposite their respective names in Schedule I bears to the aggregate number of
Firm Shares set forth opposite the names of all such non-defaulting
Underwriters, or in such other proportions as you may specify, to purchase the
Shares which such defaulting Underwriter or Underwriters agreed but failed or
refused to purchase on such date; provided that in no event shall the number of
Shares that any Underwriter has agreed to purchase pursuant to this Agreement be
increased


                                       18

pursuant to this Section 9 by an amount in excess of one-ninth of such number of
Shares without the written consent of such Underwriter. If, on the Closing Date,
any Underwriter or Underwriters shall fail or refuse to purchase Firm Shares and
the aggregate number of Firm Shares with respect to which such default occurs is
more than one-tenth of the aggregate number of Firm Shares to be purchased, and
arrangements satisfactory to you and the Company for the purchase of such Firm
Shares are not made within 36 hours after such default, this Agreement shall
terminate without liability on the part of any non-defaulting Underwriter or the
Company. In any such case either you or the Company shall have the right to
postpone the Closing Date, but in no event for longer than seven days, in order
that the required changes, if any, in the Registration Statement and in the
Prospectus or in any other documents or arrangements may be effected. If, on an
Option Closing Date, any Underwriter or Underwriters shall fail or refuse to
purchase Additional Shares and the aggregate number of Additional Shares with
respect to which such default occurs is more than one-tenth of the aggregate
number of Additional Shares to be purchased on such Option Closing Date, the
non-defaulting Underwriters shall have the option to (i) terminate their
obligation hereunder to purchase the Additional Shares to be sold on such Option
Closing Date or (ii) purchase not less than the number of Additional Shares that
such non-defaulting Underwriters would have been obligated to purchase in the
absence of such default. Any action taken under this paragraph shall not relieve
any defaulting Underwriter from liability in respect of any default of such
Underwriter under this Agreement.

      If this Agreement shall be terminated by the Underwriters, or any of them,
because of any failure or refusal on the part of the Company to comply with the
terms or to fulfill any of the conditions of this Agreement, or if for any
reason the Company shall be unable to perform its obligations under this
Agreement, the Company will reimburse the Underwriters or such Underwriters as
have so terminated this Agreement with respect to themselves, severally, for all
out-of-pocket expenses (including the fees and disbursements of their counsel)
reasonably incurred by such Underwriters in connection with this Agreement or
the offering contemplated hereunder.

      10.   Counterparts. This Agreement may be signed in two or more
counterparts, each of which shall be an original, with the same effect as if the
signatures thereto and hereto were upon the same instrument.

      11.   Applicable Law. This Agreement shall be governed by and construed in
accordance with the internal laws of the State of New York.

      12.   Headings. The headings of the sections of this Agreement have been
inserted for convenience of reference only and shall not be deemed a part of
this Agreement.


                            [SIGNATURE PAGE FOLLOWS]


                                       19

                                            Very truly yours,

                                            ONYX PHARMACEUTICALS, INC.



                                            By: /s/ Hollings C. Renton
                                                ________________________________
                                                Name: Hollings C. Renton
                                                Title: Chairman of the Board,
                                                       President and Chief
                                                       Executive Officer

Accepted as of the date hereof:

Morgan Stanley & Co. Incorporated
Lehman Brothers Inc.
SG Cowen Securities Corporation

Acting severally on behalf of themselves
      and the several Underwriters named in
      Schedule I hereto.

By: Morgan Stanley & Co. Incorporated

By: /s/ Bryan W. Andrzejewski
    ________________________________
    Name: Bryan W. Andrzejewski
    Title: Executive Director

                                                                      SCHEDULE I



                      UNDERWRITER                           NUMBER OF FIRM SHARES TO BE PURCHASED
                                                         
Morgan Stanley & Co. Incorporated.......................                  2,500,000

Lehman Brothers Inc.....................................                  1,500,000

SG Cowen Securities Corporation.........................                  1,000,000
                                                                        -----------
         Total..........................................                  5,000,000


                                                                       EXHIBIT A


                            [FORM OF LOCK-UP LETTER]



July __, 2003



Morgan Stanley & Co. Incorporated
Lehman Brothers Inc.
SG Cowen Securities Corporation
c/o  Morgan Stanley & Co. Incorporated
     1585 Broadway
     New York, NY 10036

Dear Sirs and Mesdames:

      The undersigned understands that Morgan Stanley & Co. Incorporated
("MORGAN STANLEY") proposes to enter into an Underwriting Agreement (the
"UNDERWRITING AGREEMENT") with Onyx Pharmaceuticals, Inc., a Delaware
corporation (the "COMPANY"), providing for the public offering (the "PUBLIC
OFFERING") by the several Underwriters, including Morgan Stanley (the
"UNDERWRITERS"), of ____________ shares (the "SHARES") of the common stock, par
value $0.001 per share, of the Company (the "COMMON STOCK").

      To induce the Underwriters that may participate in the Public Offering to
continue their efforts in connection with the Public Offering, the undersigned
hereby agrees that, without the prior written consent of Morgan Stanley on
behalf of the Underwriters, he or she will not, during the period commencing on
the date hereof and ending the earlier of (i) 90 days after the date of the
final Prospectus Supplement relating to the Public Offering (the "PROSPECTUS");
or (ii) August 1, 2003 if no Prospectus has been filed with the Securities and
Exchange Commission (the "SEC") by August 1, 2003: (1) offer, pledge, sell,
contract to sell, sell any option or contract to purchase, purchase any option
or contract to sell, grant any option, right or warrant to purchase, lend, or
otherwise transfer or dispose of, directly or indirectly, any shares of Common
Stock or any securities convertible into or exercisable or exchangeable for
Common Stock or (2) enter into any swap or other arrangement that transfers to
another, in whole or in part, any of the economic consequences of ownership of
the Common Stock, whether any such transaction described in clause (1) or (2)
above is to be settled by delivery of Common Stock or such other securities, in
cash or otherwise. The foregoing sentence shall not apply to (a) transactions
relating to shares of Common Stock or other securities acquired in open market
transactions after the completion of the Public Offering and (b) transfers of
shares of Common Stock or any security convertible into Common Stock as a bona
fide gift or gifts; provided that in the case of any transfer or distribution
pursuant to clause (b), (i) each donee or distributee shall execute and

deliver to Morgan Stanley a duplicate form of this Lock-up Letter and (ii) no
filing by any party (donor, donee, transferor or transferee) under Section 16(a)
of the Securities Exchange Act of 1934, as amended, shall be required or shall
be made voluntarily in connection with such transfer or distribution (other than
a filing on a Form 5 made after the expiration of the 90-day period referred to
above).

      In addition, the undersigned agrees that, without the prior written
consent of Morgan Stanley on behalf of the Underwriters, he or she will not,
during the period commencing on the date hereof and ending the earlier of 90
days after the date of the Prospectus, or August 1, 2003 if no Prospectus has
been filed with the SEC by August 1, 2003, make any demand for or exercise any
right with respect to, the registration of any shares of Common Stock or any
security convertible into or exercisable or exchangeable for Common Stock. The
undersigned also agrees and consents to the entry of stop transfer instructions
with the Company's transfer agent and registrar against the transfer of the
undersigned's share of Common Stock except in compliance with the foregoing
restrictions.

      The undersigned understands that the Company and the Underwriters are
relying upon this Lock-Up Agreement in proceeding toward consummation of the
Public Offering. The undersigned further understands that this Lock-Up Agreement
is irrevocable and shall be binding upon the undersigned's heirs, legal
representatives, successors and assigns.

      Whether or not the Public Offering actually occurs depends on a number of
factors, including market conditions. Any Public Offering will only be made
pursuant to an Underwriting Agreement, the terms of which are subject to
negotiation between the Company and the Underwriters.

                                            Very truly yours,



                                            ____________________________________
                                            (Name)

                                            ____________________________________
                                            (Address)