Filed pursuant to Rule 424(b)(3)
Reg. No. 333-92055

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.

DATED DECEMBER 8, 1999

8,140,667 SHARES

LEUKOSITE, INC.
COMMON STOCK

Selling stockholders identified in this prospectus may sell up to
8,140,667 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 26, 1999, the closing sale price of the common stock,
as reported on the Nasdaq National Market, was $38.25 per share.

INVESTING IN THE COMMON STOCK INVOLVES A HIGH DEGREE
OF RISK. SEE "RISK FACTORS," BEGINNING ON PAGE 4.

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 December 8, 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, 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
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 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 OF 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.

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 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, 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 COLLARATIVE 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. and Genentech, 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 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 $68.8
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

-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.

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 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
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.

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.

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 merger 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.

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.
An additional 631,295 shares of common stock were issued on November 30, 1999.
These shares were issued to former CytoMed shareholders as additional merger
consideration when a milestone payment was received by LeukoSite.

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
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 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 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

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

Abrams, Larry 1,320 1,320 0 *
24 Central Park South
New York, NY 10019

Atlas Venture Fund II, L.P. 74,615 74,615 0 *
222 Berkeley Street
Boston, MA 02116
Attn: Jean-Francois Formela

Atlas Venture Europe Fund B.V. 60,131 60,131 0 *
P. O. Box 5225
1410 AE NAARDEN
The Netherlands
Attn: Hans Bosman

Beck, Thomas R. 1,050 1,050 0 *
345 Silver Hill Road
Concord, MA 01742

Biotechnology Development 51,291 51,291 0 *
Fund, L.P.
575 High Street, Suite 201
Palo Alto, CA 94301
Attn: Virginia Leung

The Family Trust f/b/o James M. Casty 12 12 0 *
c/o Mr. James M. Casty
Robin Lane
Alpine, NJ 07620

The Family Trust f/b/o Lee S. Casty 44 44 0 *
c/o Mr. James M. Casty
Robin Lane
Alpine, NJ 07620

The Family Trust f/b/o Ronald G. Casty 8 8 0 *
c/o Mr. James M. Casty
Robin Lane
Alpine, NJ 07620

The Family Trust f/b/o Scott R. Casty 12 12 0 *
c/o Mr. James M. Casty
Robin Lane
Alpine, NJ 07620

Chiron Corporation 87,039 87,039 0 *
4560 Horton Street
Emeryville, CA 94608

CIP Capital, L.P. 59,454 59,454 0 *
Bldg. 300
435 Devon Park Drive
Wayne, PA 19087
Attn: Joseph Jackson

Deutsche Vermogens 100,706 100,706 0 *
Bildungsgesellschaft mbH
Feldbergstr. 22
60323 Frankfurt am Main
Germany
Attn: Berndt Ubach-Utermohl

Everest Trust 16,334 16,334 0 *
c/o Rho Asset Management Co.
767 Fifth Avenue, 43rd Floor
New York, NY 10153

Fisher, Richard A 524 524 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 37,187 37,187 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 1,653,523 1,644,524 8,999 *
44 Nassau Street
Princeton, NJ 08542

Hudson Trust 14,104 14,104 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 162,036 162,036 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. 230,816 230,816 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 563,001 563,001 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

Norma Sarofim 80 80 0 *
778 Park Avenue
New York, NY 10021

Schildkrout, Elliot & Barbara 7,777 7,777 0 *
45 Monadanock Road
Newton, MA 02167

Schroders PLC and
Affiliated Entities 878,625(6) 445,542 433,083 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. 54,712 54,712 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. 588 588 0 *
15 Edmunds Road.
Wellesley Hills, MA 02481

WPG-Farber Present 43,681 43,681 0 *
Fund, L.P.
One New York Plaza
NewYork, NY 10004-1950
Attn: Anthony Avicolli

WPG-Farber Present 2,788 2,788 0 *
Overseas, Ltd.
One New York Plaza
New York, NY 10004-1950
Attn: Anthony Avicolli

WPG-Present QP Fund, L.P. 4,820 4,820 0 *
One New York Plaza
New York, NY 10004-1950
Attn: Anthony Avicolli

YK Capital, L. P. 83,000(10) 75,000 8,000 *
509 Rochampton Road
Hillsborough, California 94010

* 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) 160,163 shares to
HealthCare Ventures III, L.P. and (b) 47,597 shares issued 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, 14,694 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
Sciences

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) 97,259 shares issued at
the closing to Schroder 1; (b) 21,613 shares issued at the closing to
Schroder 2; (c) 34,235 shares issued at the closing to the Schroder
Trust; and (d) 768 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.

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 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
DECEMBER 8, 1999 (UNLESS OTHERWISE INDICATED HEREIN). 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 3
Risk Factors 4
LeukoSite 13
Recent Developments 13
Use of Proceeds 14
Selling Stockholders 14
Plan of Distribution 22
Legal Matters 24
Experts 24

8,140,667 SHARES

LEUKOSITE, INC.

COMMON STOCK

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

PROSPECTUS

December 8, 1999

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