AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION
ON AUGUST 17, 1999
REGISTRATION NO. 333-______

SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, DC 20549
-----------

FORM S-3

REGISTRATION STATEMENT UNDER
THE SECURITIES ACT OF 1933
-----------

LEUKOSITE, INC.
(Exact name of registrant as specified in its charter)

DELAWARE 2834 04-3173859
(State or other jurisdiction of (Primary Standard Industrial (I.R.S. Employer
incorporation or organization) Classification Code Numbers) Identification No.)

215 FIRST STREET
CAMBRIDGE, MA 02142
(617) 621-9350
(Address, including zip code, and telephone number, including area
code, of registrant's principal executive offices)
-----------

WITH COPIES TO:

CHRISTOPHER K. MIRABELLI, PH.D. JUSTIN P. MORREALE, ESQ.
CHAIRMAN OF THE BOARD OF DIRECTORS JULIO E. VEGA, ESQ.
LEUKOSITE, INC. Bingham Dana LLP
215 First Street 150 Federal Street
Cambridge, MA 02142 Boston, MA 02110
(617) 621-9350 (617) 951-8000

(Name, address, including zip code, and telephone number,
including area code, of agent for service)
-----------

APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC:

FROM TIME TO TIME AFTER THIS REGISTRATION STATEMENT BECOMES EFFECTIVE

If the only securities being registered on this form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box. |_|

If any of the securities being registered on this form are to be offered
on a delayed or continuous basis pursuant to Rule 415 under the Securities Act
of 1933, check the following box. |X|

If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. |_|

If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. |_|

If delivery of the prospectus is expected to be made pursuant to Rule
434, please check the following box. |_|

CALCULATION OF REGISTRATION FEE

- ------------------------ ------------- --------------- ----------------- ------------
PROPOSED PROPOSED
MAXIMUM MAXIMUM AMOUNT OF
TITLE OF SECURITIES AMOUNT TO BE AGGREGATE PRICE AGGREGATE REGISTRATION
TO BE REGISTERED REGISTERED(1) PER UNIT(2) OFFERING PRICE(2) FEE
- ------------------------ ------------- --------------- ----------------- ------------

Common Stock,
par value $.01 per share 1,675,518 $19.125 $32,044,282 $8,909
- ------------------------ ------------- --------------- ----------------- ------------

(1) The total amount of selling shares listed in the prospectus is
9,342,525. A Registration Statement filed on October 15, 1998 registered
7,071,810 shares, of which 6,731,382 shares are being carried forward as of the
date hereof and are covered by the combined prospectus contained herein. A
registration fee of $14,874.49 was paid on October 15, 1998. A second
Registration Statement declared effective on August 6, 1999, registered an
additional 935,625 shares, all of which are being carried forward as of the date
hereof and are covered by the combined prospectus contained herein. A
registration fee of $4,033.00 was paid on April 13, 1999. The number of new
shares to be registered under this Registration Statement is 1,675,518.

(2) Estimated solely to determine the registration fee. Calculated in
accordance with Rule 457(c), based on the offering of up to 1,675,518 shares at
a purchase price of $19.125 per share, which is the average of the high and low
sale prices reported in the consolidated reporting system of the Nasdaq National
Market on August 13, 1999. It is not known how many shares will be purchased
under this Registration Statement or at what price such shares will be
purchased.

----------------------------------

THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF
THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SECTION 8(A), MAY
DETERMINE.

PROSPECTUS

THE INFORMATION IN THIS PROSPECTUS IS NOT COMPLETE AND MAY BE CHANGED.
THESE SECURITIES SHALL NOT BE SOLD UNTIL THE REGISTRATION STATEMENT FILED WITH
THE SECURITIES AND EXCHANGE COMMISSION IS EFFECTIVE. THIS PROSPECTUS IS NOT AN
OFFER TO SELL THESE SECURITIES AND IT IS NOT SOLICITING AN OFFER TO BUY THESE
SECURITIES IN ANY STATE WHERE THE OFFER OR SALE IS NOT PERMITTED.

9,342,525 SHARES

LEUKOSITE, INC.
COMMON STOCK

Selling stockholders identified in this prospectus may sell up to
9,342,525 shares of common stock of LeukoSite, Inc. LeukoSite will not receive
any of the proceeds from the sale of shares by the selling stockholders.
LeukoSite's common stock is listed on the Nasdaq National Market under the
symbol "LKST". On August 13, 1999, the closing sale price of the common stock,
as reported on the Nasdaq National Market, was $19.00 per share.

INVESTING IN THE COMMON STOCK INVOLVES A HIGH DEGREE
OF RISK. SEE "RISK FACTORS," BEGINNING ON PAGE 3.

NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES, OR DETERMINED IF
THIS PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

The selling stockholders may sell the shares of common stock described in
this prospectus in public or private transactions, on or off the National Market
System of the Nasdaq Stock Market, at prevailing market prices, or at privately
negotiated prices. The selling stockholders may sell shares directly to
purchasers or through brokers or dealers. Brokers or dealers may receive
compensation in the form of discounts, concessions or commissions from the
selling stockholders. More information is provided in the section titled "Plan
of Distribution."

The date of this prospectus is August 17, 1999

WHERE YOU CAN GET MORE INFORMATION

We are a reporting company and file annual, quarterly and current reports,
proxy statements and other information with the SEC. You may read and copy these
reports, proxy statements and other information at the SEC's public reference
rooms in Washington, DC, New York, NY and Chicago, IL. You can request copies of
these documents by writing to the SEC and paying a fee for the copying cost.
Please call the SEC at 1-800-SEC-0330 for more information about the operation
of the public reference rooms. Our SEC filings are also available at the SEC's
Web site at "http://www.sec.gov". In addition, you can read and copy our SEC
filings at the office of the National Association of Securities Dealers, Inc. at
1735 K Street, Washington, DC 20006.

The SEC allows us to "incorporate by reference" information that we file
with them, which means that we can disclose important information to you by
referring you to those other documents. The information incorporated by
reference is an important part of this prospectus, and information that we file
later with the SEC will automatically update and supersede this information. We
incorporate by reference the documents listed below and any future filings we
will make with the SEC under Section 13(a), 13(c), 14 or 15(d) of the Securities
Exchange Act of 1934:

-- Annual Report on Form 10-K/A for the year ended December 31, 1998

-- Quarterly Reports on Form 10-Q filed May 12, 1999 and
August 12, 1999.

-- Current Reports on Form 8-K filed January 5, 1999, February 26,
1999, April 6, 1999, April 27, 1999, June 24, 1999 and August 3,
1999.

-- The description of the common stock contained in LeukoSite's
Registration Statement on Form 8-A filed with the SEC under the
Securities Exchange Act of 1934.

-- You may request a copy of these filings at no cost, by writing,
telephoning or e-mailing us at the following address:

LeukoSite, Inc.
215 First Street
Cambridge, MA 02142
Attn: Investor Relations
(617) 621-9350
information@LeukoSite.com

This prospectus is part of a Registration Statement we filed with the SEC.
You should rely only on the information incorporated by reference or provided in
this prospectus. No one else is authorized to provide you with different
information. We are not making an offer of these securities in any state where
the offer is not permitted. You should not assume that the information in this
prospectus is accurate as of any date other than the date on the front of this
document.

FORWARD-LOOKING STATEMENTS

This prospectus contains forward-looking statements within the meaning of
Section 27A of the Securities Act of 1933 and Section 21E of the Securities
Exchange Act of 1934, including statements regarding LeukoSite's drug
development programs, clinical trials, receipt of regulatory approval, capital
needs, intellectual property, expectations and intentions.

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Forward-looking statements necessarily involve risks and uncertainties, and
LeukoSite's actual results could differ materially from those anticipated in the
forward-looking statements due to a number of factors, including those set forth
below under "Risk Factors" and elsewhere in this prospectus. The factors set
forth below under "Risk Factors" and other cautionary statements made in this
prospectus should be read and understood as being applicable to all related
forward-looking statements wherever they appear in this prospectus. We undertake
no obligation to update publicly any forward-looking statements for any reason,
even if new information becomes available or other events occur in the future.

RISK FACTORS

INVESTING IN LEUKOSITE'S COMMON STOCK IS VERY RISKY. YOU SHOULD BE ABLE TO
BEAR A COMPLETE LOSS OF YOUR INVESTMENT. YOU SHOULD CAREFULLY CONSIDER THE
FOLLOWING FACTORS, IN ADDITION TO THE OTHER INFORMATION IN THIS PROSPECTUS.

OUR DRUG CANDIDATES MAY NOT PROVE TO BE SAFE AND EFFECTIVE IN HUMAN
CLINICAL TRIALS.

We are currently testing three monoclonal antibody and two small molecule
products in human clinical trials. Clinical trials of drug candidates involve
the testing of potential therapeutic agents in humans to determine whether the
drug candidates are safe and effective and, if so, to what degree. Many drugs in
human clinical trials fail to demonstrate the desired safety and efficacy
characteristics. Drugs in later stages of clinical development may fail to show
the desired safety and efficacy traits despite having progressed through initial
human testing. The clinical trials of any of our drug candidates may not be
successful which may prevent us from commercializing the drug, substantially
impairing our business, financial condition and results of operations.

OUR DRUG CANDIDATES ARE SUBJECT TO GOVERNMENTAL REGULATION AND PRODUCT
APPROVALS.

Our products currently under development are subject to extensive and
rigorous regulation by the federal government, principally the FDA, and by state
and local governments. If we market these products abroad, foreign governments
may also impose export/import requirements and other restrictive regulations. We
must complete all appropriate regulatory clearance processes before
commercializing a product, which is lengthy and expensive. We cannot be sure
that we can obtain necessary regulatory approvals on a timely basis, if at all,
for any of the products we are currently developing, and all of the following
could have a material adverse effect on our business, financial condition and
results of operations

-- significant delays in obtaining or failing to obtain required
approvals

-- loss of previously obtained approvals

-- failing to comply with existing or future regulatory requirements

Complying with FDA regulatory requirements applicable to product
development and obtaining FDA approval can take a number of years, involves the
expenditure of substantial resources and is uncertain. Many products that
initially appear promising ultimately do not reach the market because they are
found to be unsafe or ineffective or cannot meet the FDA's other

-3-

regulatory requirements. Moreover, it is possible that the current regulatory
framework could change or additional regulations could arise at any stage during
our product development, which may affect our ability to obtain approval as
anticipated, delay the submission or review of an application, or require
additional expenditures by LeukoSite.

All of LeukoSite's product candidates will require FDA and foreign
government approvals for commercialization, none of which have been obtained.
LeukoSite and ILEX Oncology are required to file a Biologics Licensing
Application (BLA) with the FDA before beginning commercialization of
CAMPATH-Registered Trademark- in the United States and must also file for
marketing approval from other jurisdictions. We are not certain when,
independently or with our collaborative partners, we will submit any marketing
applications for our other monoclonal antibodies or small molecule antagonists
under development. We cannot guarantee that any studies will demonstrate that
the products are safe and effective for their intended uses, or that the FDA
will grant required approval on a timely basis, or at all, for
CAMPATH-Registered Trademark- or other product for any studied indications.

Government regulations may cause delays in LeukoSite's marketing of
products for a considerable or indefinite time, which could impose costly
procedural requirements upon LeukoSite's activities. While we attempt to comply
with such applicable government regulations, larger companies or companies more
experienced in regulatory affairs may obtain a competitive advantage over us.
Delays in obtaining governmental regulatory approval could adversely affect our
marketing strategy as well as our ability to generate revenue from commercial
sales. Our inability to obtain marketing approval of our products on a timely
basis, or at all, would have a material adverse effect on our business,
financial condition and results of operations.

WE ARE IN THE EARLY STAGES OF PRODUCT DEVELOPMENT.

Many of LeukoSite's research and development programs are at an early
stage of development. The FDA has not approved any of our product candidates. We
have limited experience in conducting preclinical and clinical trials.
Furthermore, even if we receive initially positive preclinical trial results,
such results do not mean that similar results will be obtained in the later
stages of drug development, such as additional preclinical trials or human
clinical testing. All of our potential drug candidates are prone to the risks of
failure inherent in pharmaceutical product development, including the
possibility that none of our drug candidates will or can

-- be safe and effective

-- otherwise meet applicable regulatory standards

-- receive the necessary regulatory marketing approvals

-- develop into commercially viable drugs

-- be manufactured or produced economically and on a large scale

-- be successfully marketed

-- be reimbursed by government or private consumers

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-- achieve customer acceptance

In addition, third parties may preclude us from marketing our drugs
through enforcement of their proprietary rights. Or, third parties may succeed
in marketing equivalent or superior drug products. Our failure to develop safe,
commercially viable drugs would have a material adverse effect on our business,
financial condition and results of operations.

WE DEPEND ON OUR COLLABORATIVE PARTNERS.

A key element of LeukoSite's strategy is to accelerate certain of its drug
discovery and development programs and to fund its capital requirements, in
part, through collaboration agreements with major pharmaceutical companies. We
currently have collaboration agreements with Warner-Lambert Company, Roche
Bioscience, Kyowa Hakko Kogyo, Ltd. and Genentech, Inc. Each of our partners has
the right, but not the obligation, to conduct preclinical and clinical trials of
the compounds developed during their collaboration with us and to develop and
commercialize any drug candidates resulting from their collaboration with us.
For example, under the Genentech collaboration agreement, Genentech has the
right, but not the obligation, to conduct Phase III clinical trials of our
LDP-02 monoclonal antibody product. Thus, the collaboration agreements allow our
collaborative partners significant discretion in electing whether to pursue the
development of any potential drug candidates. As a result, we cannot control the
amount and timing of the resources dedicated by our collaborative partners to
their respective collaborations with us.

This also means that LeukoSite's right to receive revenues under the
collaboration agreements for drug development milestones or royalties,
co-promotion rights or profit-sharing on sales is dependent largely upon the
activities and the development, manufacturing and marketing resources of its
collaborative partners. As such

-- our partners may not pursue the development and commercialization of
compounds resulting from their collaboration with us

-- any development or commercialization may not be successful even if
our partners pursued such efforts

-- we may not derive any royalty or product sales revenue from our
collaborations

Moreover, certain drug candidates discovered by LeukoSite may be
competitive with our partners' drugs or drug candidates. Accordingly, it is
possible that our collaborative partners will choose not to proceed with the
development of our drug candidates or that they will pursue their existing or
alternative technologies in preference to our drug candidates. We advise you
that

-- our interests may not always coincide with those of our
collaborative partners

-- some of our collaborative partners could independently develop or
develop with third parties drugs which compete with ours

-- disagreements with our partners over rights to technology or other
proprietary interests might occur, leading to delays in research or
in the development and commercialization of certain product
candidates (such disputes could also require or result in litigation
or arbitration, which is time-consuming and expensive)

-5-

OUR AGREEMENTS WITH OUR COLLABORATIVE PARTNERS ARE TERMINABLE.

LeukoSite relies on its collaborative partners to fund a substantial
portion of its research operations and clinical trials. Although each of the
collaboration agreements may be extended past its current term, we cannot be
sure that these contracts will be extended or renewed, or that any renewal, if
made, will be on terms favorable to us. The following table explains how some of
our major collaboration agreements can be terminated:

AGREEMENT TERMINATION RIGHTS
- --------------------------------------- --------------------------------------
Warner-Lambert Company (all agreements) By either party upon breach by other
party; at any time for any reason upon
6 months written notice

Kyowa Hakko Kogyo, Ltd. By either party upon breach by other
party; at any time upon 60 days
written notice

Genentech, Inc. By either party upon breach by other
party; at any time upon 9 months
written notice

Roche BioScience By either party upon breach by other
party and for such other reasons (by
Roche BioScience) as set forth in the
agreement

In short, we cannot guarantee that any of the collaboration agreements
will remain in effect for its expected term. If any of the collaborative
partners terminates or breaches its agreement with LeukoSite, or fails to
conduct its collaborative activities in a timely manner, then the development or
commercialization of any drug candidate or research program with such partner
could be delayed or terminated. Alternatively, we may have to devote unexpected
and unbudgeted additional resources to such development or commercialization,
all of which could have a material adverse effect on our business, financial
condition and results of operations.

WE HAVE A HISTORY OF LOSSES AND AN EXPECTATION OF FUTURE LOSSES.

LeukoSite has incurred a net operating loss every year since it was
incorporated in May 1992, and had an accumulated deficit of approximately $57.5
million through June 30, 1999. LeukoSite expects to incur significant additional
operating losses over the next several years and expects cumulative losses to
increase substantially due to expanded research and development efforts,
preclinical and clinical trials and commercialization expenses. In 1999, we
expect that our only revenues will come from milestone payments and any other
amounts received under existing and future collaboration agreements, if any. It
is possible that we may not be able to

-- successfully commercialize any of our products

-- establish any additional collaborative relationships on terms
acceptable to us

-6-

-- maintain the current collaboration agreements in effect

-- achieve the milestones that are required for us to receive funds
from our current collaborative partners

LeukoSite's ability to generate revenue or achieve profitability is
dependent in part on its and its collaborative partners' ability to complete the
development of drug candidates successfully, to obtain regulatory approvals for
drug candidates and to manufacture and commercialize any resulting drugs. We
cannot provide assurance that we will successfully identify, develop,
commercialize, manufacture and market any products, obtain required regulatory
approvals or achieve profitability.

WE HAVE SIGNIFICANT DEVELOPMENT COSTS AND ARE UNCERTAIN AS TO THE
AVAILABILITY OF FUTURE FUNDING.

LeukoSite will require substantial additional funds in order to finance
its drug discovery and development programs, fund operating expenses, pursue
regulatory clearances, and prosecute and defend its intellectual property
rights. We depend heavily upon our collaborative partners for research and
clinical trials funding and we cannot be sure that our existing collaboration
agreements will provide the necessary funding to meet our operating expenses.

LeukoSite intends to seek additional funding through public or private
financing or collaboration or other arrangements with collaborative partners. If
we raise additional funds by issuing new equity securities, this may result in
further dilution to our existing stockholders. In addition, any future investors
may demand (and we may have to grant) rights superior to those of our existing
stockholders in order to obtain additional funds. Still, we cannot be sure that
additional financing will be available, in the first instance, from any sources
or, even if available, that such funds will be available on acceptable terms.

OUR PATENT AND PROPRIETARY RIGHTS COULD AFFECT OUR ABILITY TO COMPETE.

LeukoSite's success will depend in part on its ability to obtain United
States and foreign patent protection for its drug candidates and processes,
preserve its trade secrets, and operate without infringing the proprietary
rights of third parties. We place considerable importance on obtaining patent
protection for significant new technologies, products and processes. Legal
standards relating to the validity of patents covering pharmaceutical and
biotechnological inventions, and the scope of claims made under such patents,
are still developing. Our patent position is highly uncertain and involves
complex legal and factual questions. We cannot be certain that the named
applicants or inventors of the subject matter covered by our patent applications
or patents (whether directly owned by us or licensed to us) were the first to
invent or the first to file patent applications for such inventions. Third
parties may challenge, infringe upon, circumvent or seek to invalidate existing
or future patents owned by or licensed to us. A court or other agency with
jurisdiction may find our patents unenforceable. Even if we have valid patents,
these patents still may not provide sufficient protection against competitive
products or other commercially valuable products or processes.

If a third party claims the same or overlapping subject matter as we have
claimed in a United States patent application or patent, then we may decide or
be required to participate in interference proceedings in the United States
Patent and Trademark Office in order to determine who invented the subject
matter first. If we lost such an interference proceeding, then we would

-7-

be deprived of the patent protection we previously sought or obtained. Indeed,
regardless of whether we win or lose in such proceedings, we would still incur
substantial costs.

In addition to patent protection, LeukoSite relies on trade secrets,
proprietary know-how, and confidentiality provisions in agreements with our
collaborative partners, employees and consultants to protect our intellectual
property. We also rely on invention assignment provisions in agreements with
employees and certain consultants. It is possible that these agreements could be
breached or that we might not have adequate remedies for any such breaches.
Third parties may learn of or independently discover our trade secrets,
proprietary know-how and intellectual property, which could have a material
adverse effect on our business, financial condition and results of operations.

LeukoSite's product candidates LDP-01, LDP-02 and CAMPATH-Registered
Trademark- are humanized monoclonal antibodies. We are aware that both the U.S.
Patent and Trademark Office and certain foreign governments have issued patents
to third parties which relate to certain humanized antibodies, products useful
for making humanized antibodies, and processes for making and using humanized
antibodies. We may choose to seek or be required to seek licenses under certain
of these patents.

We are also aware of third party applications in the United States and
abroad relating to certain humanized monoclonal antibodies, products useful for
making humanized antibodies, and processes for making and using humanized
antibodies. LeukoSite may choose to seek or be required to seek licenses under
some or all of the patents which might issue from these patent applications.

INTELLECTUAL PROPERTY LITIGATION COULD HARM OUR BUSINESS.

There is significant and widespread litigation in the pharmaceutical and
biotechnology industry regarding patents and proprietary rights. LeukoSite may
need to assert claims of infringement, enforce its patents, protect trade
secrets, know-how or other intellectual property rights, or determine the scope
or validity of proprietary rights of third parties. Conversely, we may need to
defend against claims of infringement by third parties. We cannot guarantee that
any of our patents will ultimately be held valid or that our efforts to assert
or defend any patents, trade secrets, know-how or other intellectual property
rights would be successful. Similarly, we cannot guarantee that our products or
processes will not be held to infringe the patents or other intellectual
property rights of others. Uncertainties emanating from the initiation and
continuation of any patent or related litigation could have a material adverse
effect on our business, financial condition and results of operations.

The expenses of intellectual property litigation or other similar
proceedings would be likely to be substantial, and could have a material adverse
effect on LeukoSite's business, financial condition and results of operations.
An adverse outcome in such litigation or proceeding could subject LeukoSite to
significant liabilities or require LeukoSite to cease certain activities. If any
of our present or future products or processes is alleged or determined to
infringe upon the patents or impermissibly use the intellectual property of
others, we may choose or be required to obtain licenses from third parties under
their patents or proprietary rights. We cannot guarantee that we will be able to
obtain those licenses on acceptable terms, if at all. In such event, we would be
severely restricted or prohibited from the development, manufacture and sale of
our drug candidates.

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INTENSE COMPETITION CREATES RISKS.

The biotechnology and pharmaceutical industries are intensely competitive.
LeukoSite has many competitors both in the United States and abroad, including
major, multinational pharmaceutical and chemical companies, specialized
biotechnology firms and universities and other research institutions. Many of
our competitors have greater financial and other resources, such as larger
research and development staffs and more effective marketing and manufacturing
organizations. Our competitors may succeed in developing or licensing on an
exclusive basis technologies and drugs that are more effective or less costly
than any which we are currently developing, which could render our technology
and future drug products obsolete and noncompetitive. It is possible for our
competitors to obtain FDA or other regulatory approvals for drug candidates
before we can. In general, companies that begin commercial sale of their drugs
before their competitors have a significant competitive advantage in the
marketplace, including the ability to obtain certain patent and FDA marketing
exclusivity rights that would delay our ability to market certain products. Even
if our drugs or drug products are approved for sale, we cannot assure our
ability to compete successfully with competitors' existing products or products
under development.

WE RELY ON CONTRACT MANUFACTURERS AND LACK MANUFACTURING EXPERIENCE
OURSELVES.

LeukoSite depends on third parties to manufacture its product candidates
and is aware of only a limited number of manufacturers which it believes has the
ability and capability to manufacture its drug candidates for preclinical and
clinical trials. LeukoSite and ILEX have an agreement with Boehringer Ingleheim
for the production of CAMPATH-Registered Trademark- for our clinical trials and
for any commercial sales. If we were required to transfer manufacturing
processes to other third-party manufacturers, then we could experience
significant delays in supply. If, at any time, we are unable to maintain,
develop or contract for manufacturing capabilities on acceptable terms, then our
ability to conduct preclinical and clinical trials with our drug candidates will
be adversely affected, resulting in delays in the submission of drug candidates
for regulatory approvals. We have no experience in manufacturing and we
currently lack the facilities and personnel to manufacture products in
accordance with Good Manufacturing Practices as prescribed by the FDA or to
produce an adequate supply of compounds to meet future requirements for
preclinical and clinical trials.

THERE ARE RISKS ASSOCIATED WITH THE ILEX JOINT VENTURE.

LeukoSite has entered into a joint venture with ILEX for the development
and commercialization of CAMPATH-Registered Trademark- for the treatment of
chronic lymphocytic leukemia. As part of the joint venture, we are obligated to
share fifty percent of the development costs of CAMPATH-Registered Trademark-.
If LeukoSite fails for any reason to make a required capital contribution to the
joint venture, then ILEX may gain control of the management of the joint venture
and become entitled to a greater share of the profits derived from product sales
of CAMPATH-Registered Trademark-. At the same time, if ILEX fails for any reason
to make a required capital contribution to the joint venture, then we may be
required to make additional capital contributions to the joint venture to
maintain the desired level of development activities by the joint venture. It is
possible that we may not have the resources to compensate for any failure by
ILEX to make any capital contributions in its stead and the joint venture may
not be able to continue operations with lesser funding. In addition, after the
earlier of a change in control (as defined in the joint venture agreement
between LeukoSite and ILEX) of ILEX or LeukoSite and October 2, 2000, either
company has the right to purchase the other

-9-

company's ownership of the joint venture in the event of an unresolved deadlock.
As a result, LeukoSite may never be able to recoup its investment in the joint
venture.

RAPID TECHNOLOGICAL CHANGE COULD RENDER PRODUCTS OBSOLETE.

Biotechnology and related pharmaceutical technology have undergone and are
subject to rapid and significant change. LeukoSite expects that the technologies
associated with biotechnology research and development will continue along this
rapid path. Our success will depend in large part on our ability to maintain a
competitive position in the rapidly changing environment. Because of the rapid
changes in technology, our compounds, products or processes may become obsolete
before we can recover the expenses incurred in developing such compounds,
products or processes. We cannot assure that we can maintain our technological
competitiveness.

WE DEPEND ON KEY PERSONNEL.

We believe that our ability to successfully implement our business
strategy is highly dependent on our management and scientific team. None of our
executive officers has employment agreements with us. Losing the services of one
or more of these individuals might hinder our ability to achieve our development
objectives. We are highly dependent on our ability to hire and retain qualified
scientific and technical personnel. The competition for these employees is
intense. We cannot be sure that we can continue to hire and retain the qualified
personnel needed for our business. Loss of the services of or the failure to
recruit key scientific and technical personnel could adversely affect our
business, operating results and financial condition.

CERTAIN EXISTING STOCKHOLDERS HOLD A SUBSTANTIAL PORTION OF OUR STOCK.

LeukoSite's officers, directors and principal stockholders own or control
approximately 47.7% of the outstanding common stock. As a result, these
stockholders, acting together, will have the ability to control most matters
requiring approval by the stockholders.

WE HAVE NOT DECLARED ANY DIVIDENDS.

We have never declared or paid cash dividends. We do not intend to declare
or pay any cash dividends in the foreseeable future.

LEUKOSITE

LeukoSite is a biotechnology company developing proprietary monoclonal
antibody and small molecule drugs to treat patients with cancer and
inflammatory, autoimmune and viral diseases. The mailing address and telephone
number of our principal executive office is 215 First Street, Cambridge, MA
02142 (617) 621-9350.

On July 1, 1998 LeukoSite raised approximately $11.8 million in a private
placement of common stock. LeukoSite issued approximately 1,970,000 shares of
Common Stock.

-10-

RECENT DEVELOPMENTS

On February 11, 1999 LeukoSite acquired by merger all of the issued and
outstanding capital stock of CytoMed, Inc. through the issuance initially of
935,625 shares of LeukoSite's Series A Convertible Preferred Stock, which
automatically converted into 935,625 shares of common stock as of May 25, 1999.

On July 19, 1999 LeukoSite acquired by merger all of the issued and
outstanding capital stock of ProScript, Inc. and, in connection therewith,
issued an aggregate of 187,970 shares of common stock and paid initially
$411,719 in cash.

On July 20, 1999 LeukoSite raised approximately $14.4 million in a private
placement of its common stock with two institutional investors, HealthCare
Ventures V, L.P. and Perseus Capital, LLC, at a price per share of $9.70, for an
aggregate of 1,487,548 shares of common stock.

USE OF PROCEEDS

LeukoSite will not receive any proceeds from the sale of the shares of
common stock offered by the selling stockholders hereunder.

SELLING STOCKHOLDERS

The Selling Stockholders covered by this prospectus are persons who
received LeukoSite common stock in connection with (one or more of the
following) (1) the private placement of LeukoSite's common stock which occurred
on July 1, 1998, (2) the acquisition (by merger) by LeukoSite of CytoMed, Inc.,
(3) the acquisition (by merger) by LeukoSite of ProScript, Inc., and/or (4) the
private placement of LeukoSite's common stock which occurred on July 20, 1999 or
who were entitled to have shares registered in connection with piggyback
registration rights previously granted by LeukoSite.

1998 Private Placement. Under a Registration Rights Agreement dated July
1, 1998 among LeukoSite and certain selling stockholders, we agreed to register
the LeukoSite common stock sold to those selling stockholders in the private
placement and to use our best efforts to keep the Registration Statement
effective for two years, or until all of the shares are sold under the
Registration Statement, whichever comes first.

CytoMed Merger. Under an Agreement and Plan of Merger and Reorganization
dated January 4, 1999 among LeukoSite, LeukoSite Merger Corporation, a
wholly-owned subsidiary of LeukoSite, and CytoMed, Inc., we agreed to register
the LeukoSite common stock to be issued as consideration for the merger of
CytoMed with and into LeukoSite Merger Corporation for the accounts of the
former shareholders of CytoMed. We also agreed to use our best efforts to keep
the Registration Statement effective for two years, or until all of the shares
are sold under the Registration Statement, whichever comes first.

ProScript Merger. Under an Agreement and Plan of Merger and Reorganization
dated June 22, 1999 among LeukoSite, ProScript Acquisition Co., a wholly-owned
subsidiary of LeukoSite, ProScript, Inc., HealthCare Ventures III, L.P. and
HealthCare Ventures IV, L.P., the shares of LeukoSite common stock issued in
connection therewith were required to be registered upon written notice to
LeukoSite or as a piggyback on any other registration statement being filed by
the Company.

-11-

1999 Private Placement. Under two separate Registration Rights Agreements
each dated July 20, 1999 between LeukoSite and each of Perseus Capital LLC and
HealthCare Ventures V, L.P., we agreed to register the LeukoSite common stock
issued in connection with the private placement and to use our best efforts to
keep the Registration Statement effective for two years, or until all of the
shares are sold under the Registration Statement, whichever comes first.

Our registration of the shares of common stock hereunder does not
necessarily mean that the selling stockholders will sell all or any of the
shares. The following tables set forth certain information regarding the
beneficial ownership of the common stock as of July 28, 1999, by each of the
selling stockholders. The information provided in the tables below with respect
to each selling stockholder has been obtained from such selling stockholder
and/or estimated by the Company (in cases in which such selling stockholders did
not provide updated information) based upon, where applicable, our calculation
of such ownership under the applicable merger agreement(s). Except as otherwise
disclosed below, none of the selling stockholders has, or within the past three
years has had, any position, office or other material relationship with the
Company. Because the selling stockholders may sell all or some portion of the
shares of common stock beneficially owned by them, only an estimate (assuming
each selling stockholder sells all of its shares offered hereby) can be given as
to the number of shares of common stock that will be beneficially owned by the
selling stockholders after this offering. In addition, the selling stockholders
may have sold, transferred or otherwise disposed of, or may sell, transfer or
otherwise dispose of, at any time or from time to time since the date on which
they last provided the information regarding the shares of common stock
beneficially owned by them, all or a portion of the shares of common stock
beneficially owned by them in transactions exempt from the registration
requirements of the Securities Act of 1933. Certain selling stockholders have
not provided updated information to us regarding the shares of common stock
beneficially owned by them.

-12-

SELLING STOCKHOLDERS

SHARES BENEFICIALLY
OWNED AFTER OFFERING
SHARES BENEFICIALLY NUMBER OF SHARES ------------------------
NAME AND ADDRESS OWNED PRIOR TO BEING OFFERED (2)
OF BENEFICIAL OWNER OFFERING (1) (Not subject to Escrow) NUMBER PERCENTAGE
- ------------------------------- ------------------- ----------------------- ------ ----------

Atlas Venture Fund II, L.P. 36,009 36,009 0 *
222 Berkeley Street
Boston, MA 02116
Attn: Jean-Francois Formela

Atlas Venture Europe Fund B.V. 28,981 28,981 0 *
P. O. Box 5225
1410 AE NAARDEN
The Netherlands
Attn: Hans Bosman

Beck, Thomas R. 805 805 0 *
345 Silver Hill Road
Concord, MA 01742

Biotechnology Development 39,297 39,297 0 *
Fund, L.P.
575 High Street, Suite 201
Palo Alto, CA 94301
Attn: Virginia Leung

CIP Capital, L.P. 29,276 29,276 0 *
Bldg. 300
435 Devon Park Drive
Wayne, PA 19087
Attn: Joseph Jackson

Deutsche Vermogens- 77,156 77,156 0 *
Bildungsgesellschaft mbH
Feldbergstr. 22
60323 Frankfurt am Main
Germany
Attn: Berndt Ubach-Utermohl

Fisher, Richard A 402 402 0 *
c/o UCB Research Inc.
840 Memorial Drive
Cambridge, MA 02139

Four Partners 33,334 33,334 0 *
667 Madison Avenue
7th Floor
New York, New York 10021

Gateway Venture 12,361 12,361 0 *
Partners III L.P.
8000 Maryland Avenue, Suite 1190
St. Louis, MO 63105
Attn: C.E. Anagnostopoulos, Ph.D.

Goldman Sachs & Co. 166,667 166,667 0 *
One New York Plaza
New York, New York 10004

-13-

HealthCare Ventures and
Affiliated Entities 3,184,779 (3) 3,175,779 9,000 *
44 Nassau Street
Princeton, NJ 08542

Hudson Trust 4,755 4,755 0 *
47 Hulfish Street
Suite 420
Princeton, NJ 08542
Attn: Scott Ciccone

Kisner, Daniel 5,444 5,444 0 *
1849 Montgomery Avenue
Cardiff, California 92007

Lombard Odier & Cie 425,325 425,325 0 *
Toedistrasse 36
CH8027
Zurich, Switzerland

New Day Investment
Partnership 111,111 111,111 0 *
6690 LaJolla Scenic South
LaJolla, California 92037

New York Life Insurance 79,695 79,695 0 *
Company
51 Madison Avenue, 2nd Floor
New York, NY 10010
Attn: Richard F. Drake, Room 207
2nd Floor South,
Home Office Building

Todd Noonan 500 500 0 *
International Creative
Management, Inc.
40 West 57th Street, 18th Floor
New York, New York 10019

Oracle Strategic Partners, L.P. 176,839 176,839 0 *
712 5th Avenue, 45th Floor
New York, NY 10019
Attn: Norman Schleifer

Peretz Family Investments 124,758 (4) 115,758 9,000 *
20 Larchwood Drive
Cambridge, MA 02138

Perseus Capital, LLC 1,447,595 1,447,595 0 *
The Army and Navy Club Building
1627 I Street NW
Suite 610
Washington, DC 20006

Rho Management Co. Inc. and
Affiliated Entities 559,565 (5) 559,565 0 *
767 Fifth Avenue
New York, NY 10153

Roche Finance Ltd 269,901 269,901 0 *
c/o Hoffman-La Roche, Ltd.

-14-

124 Grensacherstrasse
CH-4002 Basel
Switzerland

Schroders PLC and
Affiliated Entities 880,093 (6) 409,559 470,534 6.7%
120 Cheapside
London EC2V 6DS
ENGLAND

Springer, Timothy 781,033 (7) 770,825 10,208 *
Center for Blood Research
200 Longwood Avenue
Boston, Massachusetts 02115

Springer Family Trust 134,067 134,067 0 *
245 Walcott Road
Chestnut Hill, Massachusetts

S.R. One Ltd 222,222 222,222 0 *
200 Barr Harbor Drive
Suite 250
Four Tower Bridge
West Conshohocken, PA 19428-2977

Stiefel Laboratories Ltd. 41,918 41,918 0 *
(Ireland)
Finisklin Industrial Estate
Sligo, Ireland
Attn: Thomas J. Crowley

Walsh, Christopher 20,309 (8) 945 19,364 *
Harvard Medical School
45 Shattuck Street
Boston, Massachusetts 02115

Warner-Lambert Company 618,466 618,466 0 *
201 Tabor Road
Morris Plains, New Jersey 07950

Weiss Peck & Greer LLC and
Affiliated Entities 71,111 (9) 71,111 0 *
One New York Plaza
New York, New York 10004

Woodrich, Richard H. 451 451 0 *
15 Edmunds Road.
Wellesley Hills, MA 02481

WPG-Farber Present 37,012 37,012 0 *
Fund, L.P.
One New York Plaza
NewYork, NY 10004-1950
Attn: Anthony Avicolli

WPG-Farber Present 2,136 2,136 0 *
Overseas, Ltd.
One New York Plaza
New York, NY 10004-1950
Attn: Anthony Avicolli

WPG-Present QP Fund, L.P. 147 147 0 *

-15-

One New York Plaza
New York, NY 10004-1950
Attn: Anthony Avicolli

YK Capital, L. P. 83,000 (10) 50,000 33,000 *
509 Rochampton Road
Hillsborough, California 94010

The information provided below covers the 187,125 shares that were placed
in escrow in connection with the acquisition (by merger) by LeukoSite of
CytoMed, Inc. Any shares remaining in escrow (see table below for allocation)
shall be released after December 31, 1999, if but only if on or prior to
December 31, 1999 there have not been any claims against such escrow pursuant to
the terms of the CytoMed merger agreement, OR, after February 11, 2001, if on or
prior to December 31, 1999 there has been a claim against such escrow pursuant
to the terms of such merger agreement.

ESCROW SHARES ESCROW SHARES
BENEFICIALLY OWNED BEING OFFERED
------------------ -------------

Atlas Venture Fund II, L.P. 9,002.40 9,002.40
Atlas Venture Europe Fund B.V. 7,245.48 7,245.48
Beck, Thomas R. 201.35 201.35
BioTechnology Development 9,824.44 9,824.44
CIP Capital, L.P. 7,319.02 7,319.02
Deutsche Vermogensbild-
ungsgesellschaft mbH 19,289.22 19,289.22
Fisher, Richard A. 100.67 100.67
Gateway Venture Partners III L.P. 3,090.37 3,090.37
HealthCare Ventures III, L.P. 9,971.33 9,971.33
HealthCare Ventures IV, L.P. 3,052.94 3,052.94
Hudson Trust 1,188.81 1,188.81
New York Life Insurance Company 19,923.95 19,923.95
Oracle Strategic Partners, L.P. 44,209.97 44,209.97
Rho Management Trust II 2,814.73 2,814.73
Schroder Ventures International Life
Sciences Fund LP1 18,628.86 18,628.86
Schroder Ventures International
Life Sciences Fund LP2 4,139.77 4,139.77
Schroder Ventures International
Life Sciences Fund Trust 6,557.42 6,557.42
Schroder Ventures International
Life Sciences Co-Investment Scheme 147.45 147.45
Stiefel Laboratories Ltd. 10,479.56 10,479.56
Woodrich, Richard H. 112.84 112.84
WPG-Farber Present Fund, L.P. 9,253.22 9,253.22
WPG-Farber Present QP Fund, L.P. 36.94 36.94
WPG-Farber, Present Overseas, Ltd. 534.28 534.28
Deutsche Vermogensbild-
ungsgesellschaft mgH 19,289.22 19,289.22

-16-

- ----------
* Less than 1% of the outstanding shares of common stock (including the
shares being registered hereunder).

(1) The shares owned, and the shares included in the total number of shares
outstanding, have been adjusted, and the percentage owned has been
computed, in accordance with Rule 13d-3(d)(1) under the Securities
Exchange Act of 1934, as amended, and includes options, to the extent
called for by such rule, with respect to shares of Common Stock that can
be exercised within 60 days of July 28, 1999. The inclusion herein of any
shares as beneficially owned does not constitute an admission of
beneficial ownership of those shares. Except as set forth in the footnotes
below, such shares are beneficially owned with sole investment and sole
voting power.

(2) This column does not include shares of LeukoSite that are currently being
held in escrow in connection with and pursuant to the Agreement and Plan
of Merger and Reorganization with CytoMed, Inc.

(3) Includes shares held by HealthCare Ventures II, L.P., HealthCare Ventures
III, L.P., HealthCare Ventures IV, L.P. and HealthCare Ventures V, L.P.,
Dr. James Cavanaugh, a director of the Company, is a general partner of
the general partner of each of the foregoing listed HealthCare entities.
Includes shares issued in connection with the July 1998 private placement.
HealthCare Ventures III, L.P. and HealthCare Ventures IV, L.P. were also
represented on the Board of Directors of CytoMed, Inc. by Mark Leschly.
Includes the following shares issued in connection with the CytoMed
transaction: (a) 39,885 shares issued at the closing to HealthCare
Ventures III, L.P. and (b) 12,211 shares issued at the closing to
HealthCare Ventures IV, L.P. Includes 145,301 shares issued to HealthCare
Ventures III, L.P. and 42,669 shares issued to HealthCare Ventures IV,
L.P. in connection with the acquisition of ProScript, and 456,620 shares
issued to HealthCare Ventures V, L.P. in connection with the 1999 private
placement. Also includes 9,000 shares of Common Stock which HealthCare
Ventures LLC has the right to acquire within 60 days of July 28, 1999 upon
the exercise of stock options.

(4) Dr. Martin Peretz, a director of LeukoSite, is a general partner of the
Peretz Family Investments. Includes 9,000 shares of Common Stock which Dr.
Peretz has the right to acquire within 60 days of July 28, 1999 upon the
exercise of stock options.

(5) Includes 548,307 shares issued in connection with the July 1998 private
placement and, with respect to Rho Management Trust II, 11,258 shares
issued at the closing in connection with the CytoMed transaction.

(6) Includes shares held by Schroder Ventures International Life Sciences Fund
L.P. 1 ("Schroder 1"), Schroder Ventures International Life Science Fund
L.P.2 ("Schroder 2"), Schroder Ventures International Life Sciences Fund
Trust (the "Schroder Trust"), Schroders Incorporated and Schroder Ventures
International Life Sciences Co-Investment Scheme (the "Co-Investment
Scheme"). Includes the following shares issued in connection with the
CytoMed transaction: (a) 74,515 shares issued at the closing to Schroder;
(b) 16,559 shares issued at the closing to Schroder; (c) 26,229 shares
issued at the closing to the Schroder Trust; and (d) 589 shares issued at
the closing to the Co-Investment Scheme. Also includes 9,000 shares of
Common Stock which Schroders PLC has the right to acquire within 60 days
of July 28, 1999 upon the exercise of stock options. Schroders PLC was
represented on the Board of Directors of CytoMed, Inc. until

-17-

January 1, 1999 by Barbara Piette and is currently represented on the
LeukoSite Board of Directors by Kate Bingham.

(7) Includes shares held by Dr. Springer's wife and the Springer Family Trust.
Dr. Springer disclaims beneficial ownership of all shares owned by his
wife and beneficial ownership of the shares owned by the Springer Family
Trust except to the extent of his proportional interest. Includes 10,208
shares of Common Stock which Dr. Springer has the right to acquire within
60 days of July 28, 1999 upon the exercise of stock options. Dr. Springer
is a director of LeukoSite.

(8) Includes 9,000 shares of Common Stock which Dr. Walsh has the right to
acquire within 60 days of July 28, 1999 upon the exercise of stock
options. Dr. Walsh is a director of LeukoSite.

(9) Includes shares held by WPG Life Sciences Fund, L.P. and WPG Institutional
Life Sciences, L.P. See also WPG-Farber Present Fund, L.P., WPG-Farber
Present Overseas, Ltd. and WPG-Present QP Fund, L.P.

(10) Dr. Yasunori Kaneko, a director of LeukoSite, is a general partner of YK
Capital, L.P. Includes 8,000 shares of Common Stock which Dr. Kaneko has
the right to acquire within 60 days of July 28, 1999 upon the exercise of
stock options.

PLAN OF DISTRIBUTION

The shares of common stock may be sold from time to time by the selling
stockholders in one or more transactions at fixed prices, at market prices at
the time of sale, at varying prices determined at the time of sale or at
negotiated prices. The selling stockholders may offer their shares of common
stock in one or more of the following transactions:

-- on any national securities exchange or quotation service on which
the common stock may be listed or quoted at the time of sale,
including the Nasdaq National Stock Market,

-- in the over-the-counter market,

-- in private transactions,

-- through options,

-18-

-- by pledge to secure debts and other obligations, or

-- a combination of any of the above transactions.

If required, we will distribute a supplement to this prospectus to
describe material changes in the terms of the offering.

The shares of common stock described in this prospectus may be sold from
time to time directly by the selling stockholders. Alternatively, the selling
stockholders may from time to time offer shares of common stock to or through
underwriters, broker/dealers or agents. The selling stockholders and any
underwriters, broker/dealers or agents that participate in the distribution of
the shares of common stock may be deemed to be "underwriters" within the meaning
of the Securities Act of 1933. Any profits on the resale of shares of common
stock and any compensation received by any underwriter, broker/dealer or agent
may be deemed to be underwriting discounts and commissions under the Securities
Act of 1933.

Any shares covered by this prospectus which qualify for sale pursuant to
Rule 144 under the Securities Act of 1933 may be sold under Rule 144 rather than
pursuant to this prospectus. The selling stockholders may not sell all of the
shares. The selling stockholders may transfer, devise or gift such shares by
other means not described in this prospectus.

To comply with the securities laws of certain jurisdictions the common
stock must be offered or sold only through registered or licensed brokers or
dealers. In addition, in certain jurisdictions, the common stock may not be
offered or sold unless they have been registered or qualified for sale or an
exemption is available and complied with.

Under the Securities Exchange Act of 1934, any person engaged in a
distribution of the common stock may not simultaneously engage in market-making
activities with respect to the common stock for nine business days prior to the
start of the distribution. In addition, each selling stockholder and any other
person participating in a distribution will be subject to the Securities
Exchange Act of 1934 which may limit the timing of purchases and sales of common
stock by the selling stockholders or any such other person. These factors may
affect the marketability of the common stock and the ability of brokers or
dealers to engage in market-making activities.

All expenses of this registration will be paid by LeukoSite. These
expenses include the SEC's filing fees and fees under state securities or "blue
sky" laws. The selling stockholders will pay all underwriting discounts and
selling commissions, if any.

LEGAL MATTERS

Bingham Dana LLP, Boston, Massachusetts will give its opinion that the
shares offered in this prospectus have been validly issued and are fully paid
and non-assessable. Justin P. Morreale, a partner at Bingham Dana LLP, is the
Secretary of LeukoSite. Other attorneys at Bingham Dana LLP own a total of
approximately 1,600 shares of common stock.

EXPERTS

The consolidated financial statements of LeukoSite as of and for the year
ended December 31, 1997 and 1998, and for each of the three years in the period
ended December 31, 1998, incorporated by reference into this prospectus and this
Registration Statement, have been

-19-

audited by Arthur Andersen LLP, independent public accountants, as indicated in
their report with respect thereto, and are included herein in reliance upon the
authority of said firm as experts in giving said report.

WE HAVE NOT AUTHORIZED ANY DEALER, SALESPERSON OR OTHER PERSON TO GIVE ANY
INFORMATION OR TO MAKE ANY REPRESENTATIONS NOT CONTAINED IN THIS PROSPECTUS OR
ANY PROSPECTUS SUPPLEMENT. YOU MUST NOT RELY ON ANY UNAUTHORIZED INFORMATION.
THIS PROSPECTUS IS NOT AN OFFER OF THESE SECURITIES IN ANY STATE WHERE AN OFFER
IS NOT PERMITTED. THE INFORMATION IN THIS PROSPECTUS IS CURRENT AS OF AUGUST 17,
1999. YOU SHOULD NOT ASSUME THAT THIS PROSPECTUS IS ACCURATE AS OF ANY OTHER
DATE.

TABLE OF CONTENTS

PAGE
----

Where You Can Get More
Information.................. 2
Forward-looking Statements... 2
Risk Factors................. 3
LeukoSite.................... 10
Recent Developments.......... 11
Use of Proceeds.............. 11
Selling Stockholders......... 11
Plan of Distribution......... 18
Legal Matters................ 19
Experts...................... 19

-20-

9,342,525 SHARES

LEUKOSITE, INC.

COMMON STOCK

-------------------

PROSPECTUS

August 17, 1999

-------------------

PART II
INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION

The expenses in connection with the issuance and distribution of the
securities being registered are set forth in the following table (all amounts
except the registration fee and the listing fee are estimated):

SEC Registration Fee ......................................... $ 8,909
Nasdaq National Market Listing Fees .......................... $ 22,500
Legal Fees and Expenses ...................................... $225,000
Accountants' Fees and Expenses ............................... $ 1,500
Miscellaneous Costs .......................................... $ 300

Total .................................................. $258,209

All expenses in connection with the issuance and distribution of the
securities being offered shall be borne by the Company.

ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS

Section 145 of the Delaware General Corporation Law empowers a Delaware
corporation to indemnify its officers and directors and certain other persons to
the extent and under the circumstances set forth therein.

The Restated Certificate of Incorporation and the Amended and Restated
By-Laws of the Company, copies of which are filed herein as Exhibit 3.3 and 3.4,
provide for advancement of expenses and indemnification of officers and
directors of the Registrant and certain other persons against liabilities and
expenses incurred by any of them in certain stated proceedings and under certain
stated conditions to the fullest extent permissible under Delaware law.

Each of the Registration Rights Agreements, dated as of July 1, 1998 and
July 20, 1999 and the Agreement and Plan of Merger and Reorganization, dated
January 4, 1999, and the Agreement and Plan of Merger and Reorganization, dated
June 22, 1999 provides for indemnification by the Registrant of each of the
selling stockholders against certain liabilities under the Securities Act of
1933, the Securities Exchange Act of 1934, state securities laws or otherwise,
and provides for indemnification by the selling stockholders of the Registrant
and its directors, its officers and certain control persons against certain
liabilities under the Securities Act of 1933, the Securities Exchange Act of
1934, state securities laws or otherwise.

II-1

ITEM 16. EXHIBITS

EXHIBITS
- --------
*3.3 Restated Certificate of Incorporation of the Registrant
*3.4 Amended and Restated By-Laws of the Registrant, as amended to date
4.1 Specimen certificate for shares of common stock issued in connection with
the ProScript Merger
4.2 Specimen certificate for shares of common stock issued in connection with
the 1999 Private Placement
5 Opinion of Bingham Dana LLP
10.1 Registration Rights Agreement dated as of July 20, 1999 between LeukoSite
and Perseus Capital LLC
10.2 Registration Rights Agreement dated as of July 20, 1999 between LeukoSite
and HealthCare Ventures V, L.P.
10.3 Agreement and Plan of Merger and Reorganization dated June 22, 1999 among
LeukoSite, ProScript Acquisition Co., ProScript, Inc., HealthCare Ventures
III, L.P. and HealthCare Ventures IV, L.P.
23.1 Consent of Bingham Dana LLP (included in Exhibit 5.1)
23.2 Consent of Arthur Andersen LLP

- --------
* Incorporated by reference from the Registrant's Registration Statement on
Form S-1 (Registration No. 333-30213).

ITEM 17. UNDERTAKINGS

The undersigned registrant hereby undertakes:

(1) To file, during any period in which offers or sales are
being made pursuant to this Registration Statement, a post-effective
amendment to this Registration Statement to include any material
information with respect to the plan of distribution not previously
disclosed in this Registration Statement or any material change to
such information in this 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 such securities at
that time shall be deemed to be the initial bona 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.

II-2

The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to Section 13(a) or 15 (d) of the Securities
Exchange Act of 1934 (and, where applicable, each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the 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 initial BONA FIDE offering thereof.

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 described in Item 15 above,
or otherwise, the Registrant has been advised that in the opinion of the
Securities and Exchange Commission such indemnification is against public policy
as expressed in the Securities Act and is, therefore, unenforceable. In the
event that a claim for indemnification against such 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 such 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 such issue.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended,
the Registrant, LeukoSite, Inc., 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 Cambridge, Commonwealth
of Massachusetts, on this 17th day of August, 1999.

LEUKOSITE, INC.

/s/ Christopher K. Mirabelli
----------------------------
Christopher K. Mirabelli
CHAIRMAN OF THE BOARD OF DIRECTORS,
PRESIDENT AND CHIEF EXECUTIVE OFFICER

POWER OF ATTORNEY

Each person whose signature appears below hereby appoint each of
Christopher K. Mirabelli and Augustine Lawlor severally, acting alone and
without the other, his/her true and lawful attorney-in-fact with the authority

II-3

to execute in the name of each such person, any and all amendments (including
without limitation, post-effective amendments) to this Registration Statement on
Form S-3, to sign any and all additional registration statements relating to the
same offering of securities as this Registration Statement that are filed
pursuant to Rule 462(b) of the Securities Act, and to file such registration
statements with the Securities and Exchange Commission, together with any
exhibits thereto and other documents therewith, necessary or advisable to enable
the Registrant to comply with the Securities Act, and any rules, regulations and
requirements of the Securities and Exchange Commission in respect thereof, which
amendments may make such other changes in the Registration Statement as the
aforesaid attorney-in-fact executing the same deems appropriate.

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:

II-4

SIGNATURE TITLE DATE
--------- ----- ----

Chairman of the Board of Directors, President, August 17, 1999
/s/ Christopher K. Mirabelli Chief and Executive Officer (Principal
- -------------------------------- Executive Officer)
Christopher K. Mirabelli

Senior Vice President, Corporate August 17, 1999
/s/ Augustine Lawlor Development, Chief Operating and Financial
- -------------------------------- Office (Principal Financial and Accounting Officer)
Augustine Lawlor

Director August 17, 1999
/s/ Kate Bingham
- --------------------------------
Kate Bingham

Director August 17, 1999
/s/ James Cavanaugh
- --------------------------------
James Cavanaugh

Director August 17, 1999
/s/ Yasunori Kaneko
- --------------------------------
Yasunori Kaneko

Director August 17, 1999
/s/ Martin Peretz
- --------------------------------
Martin Peretz

Director August 17, 1999
/s/ Mark Skaletsky
- --------------------------------
Mark Skaletsky

Director August 17, 1999
/s/ Timothy A. Springer, Ph.D.
- --------------------------------
Timothy A. Springer, Ph. D.

Director August 17, 1999
/s/ Christopher T. Walsh, Ph. D.
- --------------------------------
Christopher T. Walsh, Ph. D.

II-5

EXHIBIT INDEX

EXHIBITS

*3.3 Restated Certificate of Incorporation of the Registrant
*3.4 Amended and Restated By-Laws of the Registrant, as amended to date
4.1 Specimen certificate for shares of common stock issued in connection with
the ProScript Merger
4.2 Specimen certificate for shares of common stock issued in connection with
the 1999 Private Placement
5 Opinion of Bingham Dana LLP
10.1 Registration Rights Agreement dated as of July 20, 1999 between LeukoSite
and Perseus Capital LLC
10.2 Registration Rights Agreement dated as of July 20, 1999 between LeukoSite
and HealthCare Ventures V, L.P.
10.3 Agreement and Plan of Merger and Reorganization dated June 22, 1999 among
LeukoSite, ProScript Acquisition Co., ProScript, Inc., HealthCare Ventures
III, L.P. and HealthCare Ventures IV, L.P.
23.1 Consent of Bingham Dana LLP (included in Exhibit 5.1)
23.2 Consent of Arthur Andersen LLP

- ----------
* Incorporated by reference from the Registrant's Registration Statement on
Form S-1 (Registration No. 333-30213).

II-6