LEUKOSITE INC
424B3, 1999-11-12
PHARMACEUTICAL PREPARATIONS
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                                                Filed pursuant to Rule 424(b)(3)
                                                              Reg. No. 333-85413

                                       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.

                                7,484,372  SHARES

                                 LEUKOSITE, INC.
                                  COMMON STOCK

         Selling stockholders identified in this prospectus may sell up to
7,484,372 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 November 2, 1999, the closing sale price of the common stock,
as reported on the Nasdaq National Market, was $27.25 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 November 9, 1999

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                       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, August 12, 1999,
           and November 5, 1999.

         --Current Reports on Form 8-K filed January 5, 1999, February 26, 1999,
           April 6, 1999, April 27, 1999, June 24, 1999, August 3, 1999,
           September 3, 1999, and October 21, 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
                            [email protected]

         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

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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 proposed merger
with a subsidiary of Millennium Pharmaceuticals, Inc., drug development
programs, clinical trials, receipt of regulatory approval, capital needs,
intellectual property, expectations and intentions.

         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.

         IF CAMPATH(Registered Trademark) IS NOT APPROVED FOR SALE OR IF
SCHERING AG DOES NOT SUCCESSFULLY MARKET CAMPATH(Registered Trademark) ON OUR
BEHALF, OUR ANTICIPATED FUTURE REVENUE TARGETS WILL NOT BE ACHIEVED.

         We have only one product in the process of being reviewed by the FDA
for approval, our CAMPATH(Registered Trademark) monoclonal antibody, and that
review is in the early stages. There can be no assurance that the FDA or any
foreign governmental agency will approve CAMPATH(Registered Trademark) for
commercial sale on a timely basis or at all. The failure of CAMPATH(Registered
Trademark) to be approved by FDA and foreign governmental agencies on a timely
basis would have a material adverse effect on our business, financial condition
and results of operations.

         We have no experience with commercial sales and marketing. We will rely
solely upon Schering AG and its U.S. affiliate, Berlex Laboratories, for the
marketing, distribution and sale of CAMPATH(Registered Trademark) in the United
States and Europe. We have no plans to directly market CAMPATH(Registered
Trademark). Our right to share in the profits for U.S. sales of
CAMPATH(Registered Trademark) or to receive a royalty on sales of
CAMPATH(Registered Trademark) outside the U.S. is dependent upon the marketing
and sales activities of Schering and Berlex. We can make no assurances that
there will be any profits on sales of CAMPATH(Registered Trademark) in the U.S.
In the event that there are losses resulting from the marketing of
CAMPATH(Registered Trademark) in the U.S., LeukoSite will bear one-third of
those losses.

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         Our only source of revenues from product sales, if any, for the next
several years is likely to be from sales of CAMPATH(Registered Trademark);
however, we cannot be certain that CAMPATH(Registered Trademark) will be
approved for commercial sale or accepted in the United States or in any foreign
markets. A number of factors may affect the rate and level of market acceptance
of CAMPATH(Registered Trademark), including:

     - the perception by physicians and other members of the health care
       community of its safety and efficacy or that of competing products, if
       any;

     - the effectiveness of Berlex's sales and marketing efforts in the United
       States and the effectiveness of Schering's sales and marketing efforts in
       Europe;

     - the marketing and continued development of competing drugs;

     - unfavorable publicity concerning CAMPATH(Registered Trademark) or
       comparable drugs;

     - its price relative to other drugs or competing treatments;

     - the availability of third party reimbursement; and

     - regulatory developments related to the manufacture or continued use of
       CAMPATH(Registered Trademark).

         WE RELY ON BOEHRINGER INGLEHEIM AS OUR SOLE SOURCE MANUFACTURER OF
CAMPATH(Registered Trademark) AND OUR ABILITY TO GENERATE REVENUES IN THE FUTURE
WILL BE ADVERSELY AFFECTED IF WE ARE UNABLE TO OBTAIN SUFFICIENT QUANTITIES OF
CAMPATH(Registered Trademark) FROM THEM.

         We have an agreement with Boehringer Ingleheim for the production of
CAMPATH(Registered Trademark) for our clinical trials and for any commercial
sales. If Boehringer Ingleheim were not able to produce sufficient quantities of
CAMPATH(Registered Trademark) or if we were required to transfer manufacturing
processes of CAMPATH(Registered Trademark) to other third-party manufacturers,
then we could experience significant delays in supply. We have no experience in
manufacturing and we lack the facilities and personnel to manufacture
CAMPATH(Registered Trademark) and to produce an adequate supply to meet future
requirements. The loss of Boehringer Ingleheim as our contract manufacturer for
CAMPATH(Registered Trademark) or its inability to meet our requirements of
supply of CAMPATH(Registered Trademark) would have a material adverse effect on
our business, financial condition or results of operations.

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

         We are currently testing three monoclonal antibodies and three 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

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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 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, our joint venture partner for the development of
CAMPATH(Registered Trademark), are required to file a Biologics Licensing
Application 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,

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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

         --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.

     IF OUR COLLABORATIVE PARTNERS DO NOT ACTIVELY SEEK TO COMMERCIALIZE DRUG
CANDIDATES RESULTING FROM THEIR COLLABORATION WITH US, OUR REVENUES WILL
SUFFER.

         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., Genentech,

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Inc., Schering AG, and Hoechst Marion Roussel. Warner-Lambert, Roche, Kyowa and
Hoechst Marion Roussel have the right, but not the obligation, to conduct
preclinical and clinical trials of the compounds developed during their
collaboration with us and to commercialize any drug candidates resulting from
their collaboration with us. Genentech has the right, but not the obligation, to
conduct Phase III clinical trials of and to commercialize 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 and
abilities 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 substantial or 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)

       IF OUR COLLABORATIVE PARTNERS TERMINATE OUR AGREEMENTS WITH THEM, OUR
ABILITY TO FUND OUR RESEARCH OPERATIONS AND CLINICAL TRIALS WILL SUFFER.

         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

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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.

         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.

         OUR SIGNIFICANT EXPENSES MAY CAUSE US TO CONTINUE TO INCUR 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 $68.6
million through September 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
2000, we expect that our only revenues will come primarily 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

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         -    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 revenues from product sales
or our existing collaboration agreements will provide the necessary funding to
meet our operating expenses. In the event that the merger of LeukoSite and
Millennium is not consummated, LeukoSite will have to seek additional funding
through public or private financing or collaboration or other arrangements with
collaborative partners. The raising of additional funds through the issuance of
equity securities may result in further dilution to our existing stockholders.
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.

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         WE CANNOT BE CERTAIN THAT WE WILL OBTAIN SUFFICIENT PATENT PROTECTION,
OR THAT ANY SUCH PATENTS SO OBTAINED WILL PROVIDE SUFFICIENT PROTECTION AGAINST

COMPETITIVE PRODUCTS OR PROCESSES.

         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 (PTO) in order to determine who invented the subject
matter first. If we lost such an interference proceeding, then we would 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 PTO
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

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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.

     THERE IS SIGNIFICANT AND WIDESPREAD LITIGATION IN THE PHARMACEUTICAL AND
BIOTECHNOLOGY INDUSTRY REGARDING PATENTS AND PROPRIETARY RIGHTS AND OUR BUSINESS
AND RESULTS OF OPERATIONS COULD BE ADVERSELY AFFECTED IF WE BECOME INVOLVED IN
SUCH LITIGATION.

         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.

     IF OUR COMPETITIORS SUCCEED IN DEVELOPING OR LICENSING TECHNOLOGIES OR
FUTURE DRUG PRODUCTS, OUR TECHNOLOGY AND FUTURE DRUG PRODUCTS COULD BECOME
OBSOLETE AND NONCOMPETITIVE.

         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

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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.

         In addition to relying on Boehringer Ingleheim as the sole manufacturer
of CAMPATH(Registered Trademark), 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. 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.

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       RAPID TECHNOLOGICAL CHANGE COULD RENDER OUR 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.

         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 and (617) 621-9350.

                               RECENT DEVELOPMENTS

         On October 14, 1999 LeukoSite signed an Agreement and Plan of Merger
(the "Merger Agreement") with Millennium Pharmaceuticals, Inc. and ANM, Inc., a
wholly owned subsidiary of Millennium. Under the terms of the Merger Agreement,
ANM, Inc. will merge with and into LeukoSite, whereby LeukoSite will become a
wholly-owned subsidiary of Millennium (the "Merger"). Each LeukoSite stockholder
will receive for each share of Leukosite stock 0.4296 of Millennium Common
Stock, par value $.001 per share. The Merger Agreement and Merger are subject
to, among other things, LeukoSite stockholder approval.

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         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.

         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 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.

         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.

                                 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

                                       14


<PAGE>


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 and also includes persons who were
entitled to have shares registered pursuant to piggyback registration rights
previously granted by LeukoSite to such persons.

         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.

         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 estimated information regarding
the beneficial ownership of the common stock as of November 3, 1999, by each of
the selling stockholders. The estimated information provided in the tables below
with respect to each selling stockholder, however, has not been obtained
recently from all such selling stockholders and is based only on estimations by
the Company to the best of its knowledge. 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

                                       15


<PAGE>


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. Selling stockholders have not provided updated information to us regarding
the shares of common stock beneficially owned by them.

                              SELLING STOCKHOLDERS

<TABLE>
<CAPTION>
                                                                              SHARES BENEFICIALLY
                                                                              OWNED AFTER OFFERING
                                                                              --------------------
                                 SHARES BENEFICIALLY    NUMBER OF SHARES
NAME AND ADDRESS                 OWNED PRIOR TO         BEING OFFERED
OF BENEFICIAL OWNER              OFFERING (1)
                                                                              NUMBER   PERCENTAGE
- -------------------------------  -------------------   ------------------     ------   -----------
<S>                              <C>                   <C>                    <C>      <C>
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
</TABLE>

                                       16

<PAGE>

<TABLE>
<CAPTION>
                                                                              SHARES BENEFICIALLY
                                                                              OWNED AFTER OFFERING
                                                                              --------------------
                                 SHARES BENEFICIALLY    NUMBER OF SHARES
NAME AND ADDRESS                 OWNED PRIOR TO         BEING OFFERED
OF BENEFICIAL OWNER              OFFERING (1)
                                                                              NUMBER   PERCENTAGE
- -------------------------------  -------------------   ------------------     ------   -----------
<S>                              <C>                   <C>                    <C>      <C>
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

HealthCare Ventures and
 Affiliated Entities                    1,497,759(2)      1,488,760          8,999        *
 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
</TABLE>

                                       17

<PAGE>


<TABLE>
<CAPTION>
                                                                              SHARES BENEFICIALLY
                                                                              OWNED AFTER OFFERING
                                                                              --------------------
                                 SHARES BENEFICIALLY    NUMBER OF SHARES
NAME AND ADDRESS                 OWNED PRIOR TO         BEING OFFERED
OF BENEFICIAL OWNER              OFFERING (1)
                                                                              NUMBER   PERCENTAGE
- -------------------------------  -------------------   ------------------     ------   -----------
<S>                              <C>                   <C>                    <C>      <C>
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                 124,757(3)        115,758          8,999        *
 20 Larchwood Drive
 Cambridge, MA 02138

Perseus Capital, LLC                    1,447,595(4)      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           559,565              0        *
 767 Fifth Avenue
 New York, NY 10153

Roche Finance Ltd                         269,901           269,901              0        *
 c/o Hoffman-La Roche, Ltd.
 124 Grensacherstrasse
 CH-4002 Basel
 Switzerland

Schildkrout, Elliot & Barbara               7,777             7,777              0        *
 45 Monadanock Road
 Newton, MA 02167
</TABLE>
                                       18

<PAGE>

<TABLE>
<CAPTION>
                                                                              SHARES BENEFICIALLY
                                                                              OWNED AFTER OFFERING
                                                                              --------------------
                                 SHARES BENEFICIALLY    NUMBER OF SHARES
NAME AND ADDRESS                 OWNED PRIOR TO         BEING OFFERED
OF BENEFICIAL OWNER              OFFERING (1)
                                                                              NUMBER   PERCENTAGE
- -------------------------------  -------------------   ------------------     ------   -----------
<S>                              <C>                   <C>                    <C>      <C>
Schroders PLC and
 Affiliated Entities                      842,642(6)        409,559        433,083        2.9%
 120 Cheapside
 London EC2V 6DS
 ENGLAND

Springer, Timothy                         775,547(7)        759,714         15,833        *
 Center for Blood Research
 200 Longwood Avenue
 Boston, Massachusetts 02115

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

Springer, Dr. Asa & Patricia               11,111            11,111              0        *
 3221 Diablo Trail
 Shingle Springs, CA 95682

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                         19,364(8)          9,145         10,219        *
 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
</TABLE>

                                       19


<PAGE>

<TABLE>
<CAPTION>
                                                                              SHARES BENEFICIALLY
                                                                              OWNED AFTER OFFERING
                                                                              --------------------
                                 SHARES BENEFICIALLY    NUMBER OF SHARES
NAME AND ADDRESS                 OWNED PRIOR TO         BEING OFFERED
OF BENEFICIAL OWNER              OFFERING (1)
                                                                              NUMBER   PERCENTAGE
- -------------------------------  -------------------   ------------------     ------   -----------
<S>                              <C>                   <C>                    <C>      <C>
WPG-Farber Present                         33,466            33,466              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.                   3,693             3,693              0        *
 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
</TABLE>

                                       20


<PAGE>


*      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 November 3, 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)    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. Includes 8,999 shares of Common Stock which
       HealthCare Ventures LLC has the right to acquire within 60 days of
       November 3, 1999 upon the exercise of stock options.


(3)    Dr. Martin Peretz, a director of LeukoSite, is a general partner of the
       Peretz Family Investments. Includes 8,999 shares of Common Stock which
       Dr. Peretz has the right to acquire within 60 days of November 3, 1999
       upon the exercise of stock options.

(4)    Includes 416,667 shares issued in connection with the July 1998 Private
       Placement and 1,030,928 shares issued in connection with the July 1999
       Private Placement.

(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.

                                      -21-


<PAGE>



(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 1; (b) 16,559 shares issued at the closing to
       Schroder 2; (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 8,999 shares of Common Stock which Schroders PLC
       has the right to acquire within 60 days of November 3, 1999 upon the
       exercise of stock options. Schroders PLC was represented on the board of
       directors of CytoMed, Inc. until January 1, 1999 by Barbara Piette and
       is currently represented on the LeukoSite board of directors by Kate
       Bingham.



(7)    Includes 15,833 shares of Common Stock which Dr. Springer has the right
       to acquire within 60 days of November 3, 1999 upon the exercise of stock
       options. Dr. Springer is a director of LeukoSite.

(8)    Includes 10,219 shares of Common Stock which Dr. Walsh has the right to
       acquire within 60 days of November 3, 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 November 3, 1999 upon the exercise
       of stock options.

                                      -22-


<PAGE>




                              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,

      --    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 supplement will set
forth the aggregate number of shares of common stock being offered and the terms
of such offering, including the name or names of the broker/dealers or agents,
any discounts, commissions and other terms constituting compensation from the
selling stockholders and any discounts, commissions or concessions allowed or
reallowed to be paid to broker/dealers.

        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
broker/dealers or agents. The selling stockholders and any 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

                                      -23-


<PAGE>




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 applicable rules and regulations under the Securities Exchange
Act of 1934, any person engaged in a distribution of the common stock offered
hereby may not simultaneously engage in market-making activities with respect to
the common stock for a specified period 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, including
Regulation M, 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 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 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 NOVEMBER
9, 1999 (UNLESS OTHERWISE INDICATED HEREIN). YOU SHOULD NOT ASSUME THAT THIS
PROSPECTUS IS ACCURATE AS OF ANY OTHER DATE.

         TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                      PAGE
                                      ----
<S>                                   <C>
Where You Can Get More
Information                             2
Forward-looking Statements              3
Risk Factors                            3
LeukoSite                              13
Recent Developments                    13
Use of Proceeds                        14
Selling Stockholders                   14
Plan of Distribution                   23
Legal Matters                          24
Experts                                24
</TABLE>



                                      -24-

<PAGE>

                                7,484,372 SHARES

                                 LEUKOSITE, INC.

                                  COMMON STOCK

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

                                   PROSPECTUS

                                November 9, 1999

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



                                      -25-


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