LEUKOSITE, INC.

                         SECURITIES PURCHASE AGREEMENT


      This SECURITIES PURCHASE AGREEMENT is dated as of the 18th day of
December, 1997 (this "Agreement") by and between LEUKOSITE, INC., a Delaware
corporation with its principal office at 215 First Street, Cambridge,
Massachusetts 02142 (the "Company"), and GENENTECH, INC., a Delaware
corporation with its principal office at 1 DNA Way, South San Francisco,
California 94080 (the "Purchaser").

      WHEREAS, the Company desires to issue and sell to the Purchaser an
aggregate of up to 336,135 shares (the "Common Shares") of the authorized but
unissued shares of common stock, $.01 par value per share, of the Company (the
"Common Stock"), and desires to issue to the Purchaser a Common Stock Purchase
Warrant exercisable for 250,000 shares of Common Stock (the "Warrant Shares");
and

      WHEREAS, the Purchaser, wishes to purchase the Common Shares and to
acquire such Common Stock Purchase Warrant on the terms and subject to the
conditions set forth in this Agreement.

      NOW THEREFORE, in consideration of the mutual agreements,
representations, warranties and covenants herein contained, the parties hereto
agree as follows:

      1. Definitions. As used in this Agreement, the following terms shall have
the following respective meanings:

      (a) "Affiliate" of a Person means any other Person controlled by,
controlling or under common control with such Person.

      (b) "Closing" shall have the meaning thereto ascribed in Section 2.4
hereof.

      (c)  "Closing Date" means the date of the Closing.

      (d) "Collaboration Agreement" shall mean that certain Development
Collaboration and Licensing Agreement, dated as of the date hereof, between the
Company and the Purchaser, as amended and in effect from time to time.




      (e) "Common Shares" shall have the meaning ascribed thereto in the
preamble hereto.

      (f) "Controlled Affiliate" of a Person means any other Person that is
directly or indirectly controlled by such Person.

      (g) "Conversion Common Shares" shall mean those shares of Common Stock
issued or issuable upon conversion of (i) any or all Development Loans and/or
Profit-Sharing Option Loans outstanding at any time and from time to time or
(ii) any and all Conversion Preferred Shares outstanding at any time and from
time to time.

      (h) "Conversion Preferred Shares" shall mean those shares of any series
of preferred stock, $0.01 par value per share, of the Company that are issued
upon conversion of any or all Development Loans and/or Profit-Sharing Option
Loans outstanding
at any time and from time to time.

      (i) "Conversion Shares" shall mean the Conversion Common Shares and the
Conversion Preferred Shares, collectively.

      (j) "Development Loans" shall have the meaning ascribed to such term in
the Loan Agreement.

      (k) "Exchange Act" means the Securities Exchange Act of 1934, as amended.

      (l) "Loan Agreement" shall mean that certain Loan Agreement, dated as of
the date hereof, between the Company and the Purchaser, as amended and in
effect from time to time.

      (m) "Person" shall mean an individual, partnership, limited liability
company, corporation, association, trust, joint venture, unincorporated
organization, and any government, governmental department or agency or
political subdivision thereof.

      (n) "Prohibited Transferee" shall mean any Person that, immediately after
the acquisition of any Shares or any interest therein by such Person, its
Affiliates or nominees, directly or indirectly owns or holds of record or
beneficially securities of the Company representing (or convertible into or
exercisable or exchangeable for other securities of the Company that represent)
at least fifteen percent (15%) of the voting power of all voting securities of
the Company that are outstanding immediately after such acquisition.

      (o) "Profit-Sharing Option Loans" shall have the meaning ascribed to such
term in the Loan Agreement.




      (p) "Registration Rights Agreement" shall mean that certain Registration
Rights Agreement, dated as of the date hereof, between the Company and the
Purchaser, as amended and in effect from time to time.

      (q) "Rule 144" shall mean Rule 144 promulgated under the Securities Act
and any successor or substitute rule, regulation or law.

      (r)  "SEC" shall mean the Securities and Exchange Commission.

      (s) "Securities" shall mean, collectively the Shares and the Warrant.

      (t) "Securities Act" shall mean the Securities Act of 1933, as amended.

      (u) "Shares" shall mean, collectively, the Common Shares, the Warrant
Shares and the Conversion Shares.

      (v) "Warrant" shall have the meaning ascribed to such term in Section 2.2
hereof.

      (w) "Warrant Shares" shall mean those shares of capital stock of the
Company issued or issuable upon exercise of the Warrant.

      2. Purchase and Sale of Common Shares; Issuance of Common Stock Purchase
Warrant.

      2.1 Purchase and Sale. Subject to and upon the terms and conditions set
forth in this Agreement, the Company agrees to issue and sell to the Purchaser,
and the Purchaser hereby agrees to purchase from the Company, at the Closing,
336,135 shares of Common Stock, at a purchase price of $11.90 per share. The
aggregate purchase price payable by the Purchaser to the Company for all of the
Common Shares shall be $4,000,000.

      2.2 Issuance of Common Stock Purchase Warrants. Subject to and upon the
terms and conditions set forth in this Agreement, the Company agrees to issue
to the Purchaser, at the Closing, a Common Stock Purchase Warrant,
substantially in the form attached as Exhibit A hereto (as amended and in
effect from time to time, the "Warrant"), exercisable for 250,000 shares of
Common Stock, at an exercise price of $16.22 per share. No additional
consideration shall be payable by the Purchaser in respect of the issuance by
the Company of the Warrant to the Purchaser at the Closing.

      2.3 Reservation of Warrant Shares. Prior to the Closing Date, the Company
will have duly authorized and reserved, free of preemptive rights and other



preferential rights, an aggregate of 250,000 shares of Common Stock for
issuance upon exercise of the Warrant.

      2.4 Closing. The closing of the transactions contemplated under this
Agreement (the "Closing") shall take place at the offices of Bingham Dana LLP,
150 Federal Street, Boston, Massachusetts 02110 at 1:00 p.m. on December 18,
1997 or at such other location, date and time as may be mutually agreed upon by
the Purchaser and the Company. At the Closing, the Company shall deliver to the
Purchaser (i) a single stock certificate representing the number of shares of
Common Stock purchased by the Purchaser, against payment of the purchase price
therefor by wire transfer of immediately available funds to such account or
accounts as the Company shall designate in writing and (ii) a duly executed
original of the Warrant issued by the Company to the Purchaser.

      3. Representations and Warranties of the Company. The Company hereby
represents and warrants to the Purchaser as follows:

      3.1 Incorporation. The Company is a corporation duly organized, validly
existing and in good standing under the laws of the State of Delaware and is
qualified to do business in each jurisdiction in which the character of its
properties or the nature of its business requires such qualification, except
where the failure to so qualify would not have a material adverse effect upon
the Company. The Company has all requisite corporate power and authority to
carry on its business as now conducted.

      3.2 Capitalization. The authorized capital stock of the Company consists
of (i) 25,000,000 shares of Common Stock, of which 9,538,692 shares are
outstanding on the date hereof (without giving effect to any of the
transactions contemplated hereby), (ii) 5,000,000 shares of preferred stock,
$0.01 par value per share, of which no shares are outstanding on the date
hereof and (iii) outstanding options and warrants to purchase 1,343,391 shares
of Common Stock.

      3.3 Authorization. Except as set forth on Schedule 3.3 hereto, all
corporate action on the part of the Company that is necessary for the due
authorization, execution, delivery and performance of this Agreement, the
Registration Rights Agreement, the Warrant and the Loan Agreement and the
consummation of the transactions contemplated herein and therein has been
taken. When executed and delivered by the Company, each of this Agreement, the
Registration Rights Agreement, the Warrant and the Loan Agreement shall
constitute the legal, valid and binding obligation of the Company, enforceable
against the Company in accordance with its terms, except as such may be limited
by bankruptcy, insolvency, reorganization or other laws affecting creditors'
rights generally and by general equitable principles. The Company has all
requisite corporate power to enter into this Agreement, the Registration Rights



Agreement, the Warrant and the Loan Agreement and to carry out and perform its
obligations under the terms of this Agreement, the Registration Rights
Agreement, the Warrant and the Loan Agreement.

      3.4 Valid Issuance of the Common Shares, Warrant Shares and Conversion
Shares. The Common Shares being purchased by the Purchaser hereunder will, upon
issuance pursuant to the terms hereof, be duly authorized and validly issued,
fully paid and nonassessable. The reservation, issuance, sale and delivery by
the Company of the Warrant Shares have been duly authorized by all requisite
corporate action of the Company, and the Warrant Shares have been duly reserved
in accordance with Section 2.3 of this Agreement. The Warrant Shares, upon
issuance pursuant to the terms of the Warrant, will be duly authorized and
validly issued, fully paid and nonassessable. Except as set forth in Schedule
3.4 hereto, the issuance, sale and delivery by the Company of the Conversion
Shares have been duly authorized by all requisite corporate action of the
Company, and the Conversion Shares, when issued in accordance with the terms of
the Loan Agreement or the terms of the Conversion Preferred Shares, as the case
may be, will be duly authorized and validly issued, fully paid and
nonassessable.

      3.5 Financial Statements. The Company has furnished or prior to the
Closing will furnish to the Purchaser its unaudited Statements of Income,
Stockholders' Equity and Cash Flows for the period from January 1, 1997 to
September 30, 1997, and its unaudited Balance Sheet dated September 30, 1997.
All such financial statements are hereinafter referred to collectively as the
"Financial Statements". The Financial Statements have been prepared in
accordance with generally accepted accounting principles applied on a
consistent basis during the periods involved, except for the absence of
footnotes normally contained therein and except that the Financial Statements
may be subject to normal year-end audit adjustments which, individually and in
the aggregate, will not be material. The Financial Statements fairly present,
in all material respects, the financial position of the Company and the results
of its operations as of the date and for the periods indicated thereon.

      3.6 Consents. Except as set forth in Schedule 3.6 hereto, all consents,
approvals, orders and authorizations required on the part of the Company in
connection with the execution, delivery or performance of this Agreement, the
Registration Rights Agreement, the Warrant and the Loan Agreement and the
consummation of the transactions contemplated hereby and thereby have been
obtained and will be effective as of the Closing Date.

      3.7 Brokers or Finders. The Company has not dealt with any broker or
finder in connection with the transactions contemplated by this Agreement, and
the Company has not incurred, and shall not incur, directly or indirectly, any
liability for any brokerage or finders' fees or agents' commissions or any



similar charges in connection with this Agreement or any transaction
contemplated hereby.

      3.8 Nasdaq National Market. The Common Stock is listed on the Nasdaq
National Market System, and there are no proceedings to revoke or suspend such
listing.

      3.9. SEC Documents. The Common Stock of the Company is registered
pursuant to Section 12(g) of the Exchange Act and the Company has filed all
reports, schedules, forms, statements and other documents required to be filed
by it with the SEC pursuant to the reporting requirements of the Exchange Act,
including material filed pursuant to Section 13(a) or 15(d), in addition to one
or more registration statements and amendments thereto heretofore filed by the
Company with the SEC pursuant to the Securities Act (all of the foregoing,
including filings incorporated by reference therein, being referred to herein
as the "SEC Documents"). The Company has delivered or made available to the
Purchaser true and complete copies of all SEC Documents (including, without
limitation, proxy information and solicitation materials and registration
statements) filed with the SEC. As of their respective dates, none of the SEC
Documents contained any untrue statement of a material fact or omitted to state
a material fact required to be stated therein or necessary in order to make the
statements therein, in light of the circumstances under which they were made,
not misleading.

      3.10 No Material Adverse Change. Since the date the registration
statement relating to the Company's initial public offering was filed with the
SEC, no material adverse event has occurred with respect to the Company or its
subsidiaries, except as otherwise disclosed or reflected in other SEC Documents
prepared through or as of a date subsequent to such filing.


      3.11 No Undisclosed Liabilities. The Company and its direct and indirect
subsidiaries have no liabilities or obligations incurred since September 30,
1997, except for liabilities or obligations incurred in the ordinary course of
business or liabilities or obligations which, individually or in the aggregate,
do not or would not have a material adverse effect on the financial condition
or business of the Company and its direct or indirect subsidiaries.

      3.12 No Undisclosed Events or Circumstances. No event or circumstance has
occurred or exists with respect to the Company or its direct or indirect
subsidiaries or their respective businesses, properties, prospects, operations
or financial condition, which, under applicable law, rule or regulation,
requires public disclosure or announcement by the Company but which has not
been so publicly announced or disclosed.




      3.13.Intellectual Property. The Company (and/or its wholly-owned
subsidiaries) owns or has licenses to use certain patents, copyrights and
trademarks ("intellectual property") associated with its business. To the best
of the Company's knowledge, the Company and its subsidiaries own or have the
right to use all intellectual property rights which are needed to conduct the
business of the Company and its subsidiaries as it is now being conducted. The
Company and its subsidiaries have no reason to believe that the intellectual
property rights which it owns are invalid or unenforceable or that the use of
such intellectual property by the Company or its subsidiaries infringes upon or
conflicts with any right of any third party, and neither the Company nor any of
its subsidiaries has received notice of any such infringement or conflict. The
Company and its subsidiaries have no knowledge of any infringement of its
intellectual property by any third party.


      3.14.No Litigation. No litigation against the Company or any of its
subsidiaries is pending or, to the Company's knowledge, threatened.

      3.15.Compliance With Applicable Laws and Other Instruments;
Non-Contravention. The business and operations of the Company have been and are
being conducted in accordance with all applicable laws, rules and regulations
of all governmental authorities, except for such violations of applicable laws,
rules and regulations which would not, individually or in the aggregate, have a
material adverse effect on the financial condition or business of the Company.
Neither the execution and delivery of, nor the performance of or compliance
with, this Agreement, the Registration Rights Agreement, the Warrant and the
Loan Agreement, and the transactions contemplated hereby and thereby, will,
with or without the giving of notice or passage of time, (i) result in any
breach of, or constitute a default under, or result in the imposition of any
lien or encumbrance upon any asset or property of the Company pursuant to any
agreement or other instrument to which the Company is a party or by which it or
any of its properties, assets or rights is bound or affected, except for any
such breach or default or the imposition of any such lien or encumbrance which,
either individually or in the aggregate, would not have a material adverse
effect on the financial condition or business of the Company or (ii) violate
the Certificate of Incorporation or By-Laws of the Company, or, except as set
forth in Schedule 3.15 hereto any law, rule, regulation, judgment order or
decree applicable to the Company (assuming, for this purpose, that any filings
required under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as
amended, are made). The Company is not in violation of its Certificate of
Incorporation or By-Laws nor in violation of, or in default under, any lien,
indenture, mortgage, lease, agreement, instrument, commitment or arrangement,
except for such violations or defaults which would not, individually or in the
aggregate, have a material adverse effect on the financial condition or
business of the Company. The Company is not subject to any restriction in its



Certificate of Incorporation or By-Laws or in any agreement to which the
Company is party that would prohibit the Company from entering into or
performing its obligations under this Agreement, the Registration Rights
Agreement, the Warrant and the Loan Agreement.

      3.16 Securities Laws; Governmental Approvals. Based in part upon the
representations and warranties of the Purchaser contained in Section 4 of this
Agreement, the offer, sale, issuance and delivery to Purchaser of the Shares as
contemplated by this Agreement are exempt from the registration requirements of
the Securities Act and from the registration or qualification requirements of
the securities laws of any applicable state or other U.S. jurisdiction. Based
in part on the representations and warranties of the Purchaser contained in
Section 4 of this Agreement, no authorization, consent, approval, license,
exemption of or filing or registration with any court or governmental
instrumentality, domestic or foreign, under any applicable laws, rules or
regulations is or will be necessary for or in connection with the offer, sale,
issuance and delivery by the Company to Purchaser of the Shares as contemplated
by this Agreement or for the performance by the Company of its obligations
under this Agreement, the Registration Rights Agreement, the Warrant and the
Loan Agreement, except for (i) filings under applicable securities laws which
will be made by the Company (or in the case of Section 13 of the Exchange Act,
by the Purchaser) within the prescribed periods, (ii) any filings that may be
required under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as
amended, and (iii) except as set forth in Schedule 3.16 hereto.

      4. Representations and Warranties of the Purchasers. The Purchaser
represents and warrants to the Company as follows:

      4.1 Incorporation. The Purchaser is a corporation duly organized, validly
existing and in good standing under the laws of the State of Delaware. The
Company has all requisite corporate power and authority to carry on its
business as now conducted.

      4.2 Authorization. All corporate action on the part of the Purchaser that
is necessary for the due authorization, execution, delivery and performance of
this Agreement, the Registration Rights Agreement and the Loan Agreement and
the consummation of the transactions contemplated herein and therein has been
taken. When executed and delivered, each of this Agreement, the Registration
Rights Agreement and the Loan Agreement will constitute the legal, valid and
binding obligation of the Purchaser, enforceable against the Purchaser in
accordance with its terms, except as such may be limited by bankruptcy,
insolvency, reorganization or other laws affecting creditors' rights generally
and by general equitable principles. The Purchaser has all requisite corporate
power to enter into each of this Agreement, the Registration Rights Agreement



and the Loan Agreement and to carry out and perform its obligations under the
terms of this Agreement, the Registration Rights Agreement and the Loan
Agreement.

      4.2 Purchase Entirely for Own Account. The Purchaser is acquiring the
Common Shares and the Warrant for investment for its own account and not for
resale or with a view to distribution of all or any portion of such Securities
in violation of the Securities Act. In the event that the Purchaser acquires
any Conversion Shares or any Warrant Shares, the Purchaser will be acquiring
such Conversion Shares or such Warrant Shares, as the case may be, for
investment for its own account and not for resale or with a view to
distribution of all or any portion of such Securities in violation of the
Securities Act.

      4.3 Accredited Investor Status; Etc. The Purchaser certifies and
represents to the Company that it is an "Accredited Investor" as defined in
Rule 501 of Regulation D promulgated under the Securities Act and was not
organized for the purpose of acquiring any of the Securities. The Purchaser's
financial condition is such that it is able to bear the risk of holding the
Securities for an indefinite period of time and the risk of loss of its entire
investment. The Purchaser has been afforded the opportunity to ask questions of
and receive answers from the management of the Company concerning this
investment and has sufficient knowledge and experience in investing in
companies similar to the Company in terms of the Company's stage of development
so as to be able to evaluate the risks and merits of its investment in the
Company.

      4.4 Securities Not Registered. The Purchaser understands that the
Securities have not been registered under the Securities Act, by reason of
their issuance by the Company in a transaction exempt from the registration
requirements of the Securities Act, and that the Securities must continue to be
held by the Purchaser unless a subsequent disposition thereof is registered
under the Securities Act or is exempt from such registration. The Purchaser
understands that the exemptions from registration afforded by Rule 144 (the
provisions of which are known to it) promulgated under the Securities Act
depend on the satisfaction of various conditions, and that, if applicable, Rule
144 may afford the basis for sales only in limited amounts.

      4.5 No Conflict. The execution and delivery of this Agreement, the
Registration Rights Agreement and the Loan Agreement by the Purchaser and the
consummation of the transactions contemplated hereby and thereby will not
conflict with or result in any violation of or default (with or without notice
or lapse of time, or both) under, or give rise to a right of termination,
cancellation or acceleration of any obligation or to a loss of a material
benefit under (i) any provision of the Certificate of Incorporation or By-Laws
of the Purchaser or (ii) any agreement or instrument, permit, franchise,



license, judgment, order, statute, law, ordinance, rule or regulation,
applicable to the Purchaser or its properties or assets.

      4.6 Brokers. The Purchaser has not dealt with any broker or finder in
connection with the transactions contemplated by this Agreement, and the
Purchaser has not incurred, and shall not incur, directly or indirectly, any
liability for any brokerage or finders' fees or agents' commissions or any
similar charges in connection with this Agreement or any transaction
contemplated hereby.

      4.7 Consents. Except as set forth in Schedule 4.7 hereto, all consents,
approvals, orders and authorizations required on the part of the Purchaser in
connection with the execution, delivery or performance of this Agreement, the
Registration Rights Agreement and the Loan Agreement and the consummation of
the transactions contemplated hereby and thereby have been obtained and are
effective as of the Closing Date.

      4.8 Investment Representations. The Purchaser represents and warrants to
the Company that (i) it is a Delaware corporation, (ii) was not formed for the
specific purposes of acquiring any of the Securities, (iii) it has assets in
excess of $5,000,000 and (iv) its principal office is in South San Francisco,
California.

      5.   Conditions Precedent.

      5.1. Conditions to the Obligation of the Purchaser to Consummate the
Closing. The obligation of the Purchaser to consummate the Closing and to
purchase and pay for the Common Shares at the Closing is subject to the
satisfaction of the following conditions precedent:

           (a) The representations and warranties contained herein of the
Company shall be true and correct in all material respects on and as of the
Closing Date with the same force and effect as though made on and as of the
Closing Date.

           (b) The Registration Rights Agreement, the Loan Agreement, the
Warrant and the Collaboration Agreement shall have been duly executed and
delivered by the Company.

           (c) The Company shall have performed all obligations herein required
to be performed or observed by the Company on or prior to the Closing Date.

           (d) No proceeding challenging this Agreement or the transactions
contemplated hereby, or seeking to prohibit, alter, prevent or materially delay



the Closing, shall have been instituted before any court, arbitrator or
governmental body, agency or official and shall be pending.

           (e) The acquisition of any of the Securities by the Purchaser shall
not be prohibited by any law or governmental order or regulation. All necessary
consents, approvals, licenses, permits, orders and authorizations of, or
registrations, declarations and filings with, any governmental or
administrative agency or of any other person with respect to any of the
transactions contemplated under this Agreement to be consummated at the Closing
shall have been duly obtained or made and shall be in full force and effect.

           (f) All instruments and corporate proceedings in connection with the
transactions contemplated by this Agreement to be consummated at the Closing
shall be reasonably satisfactory in form and substance to the Purchaser, and
the Purchaser shall have received copies (executed or certified, as may be
appropriate) of all documents which the Purchaser may have reasonably requested
in connection with such transactions.

           (g) The Purchaser shall have received from Bingham Dana LLP, counsel
to the Company, an opinion addressed to it, dated the Closing Date and
substantially in the form of Exhibit B hereto.

      5.2. Conditions to the Obligation of the Company to Consummate the
Closing. The obligation of the Company to consummate the Closing and to issue
to the Purchaser the Common Shares and the Warrant at the Closing is subject to
the satisfaction of the following conditions precedent:

           (a) The representations and warranties contained herein of the
Purchaser shall be true and correct in all material respects on and as of the
Closing Date with the same force and effect as though made on and as of the
Closing Date.

           (b) The Registration Rights Agreement, the Loan Agreement, and the
Collaboration Agreement shall have been duly executed and delivered by the
Purchaser.

           (c) The Purchaser shall have performed all obligations herein
required to be performed or observed by the Purchaser on or prior to the
Closing Date.

           (d) No proceeding challenging this Agreement or the transactions
contemplated hereby, or seeking to prohibit, alter, prevent or materially delay
the Closing, shall have been instituted before any court, arbitrator or
governmental body, agency or official and shall be pending.




           (e) The sale and/or issuance of the Securities by the Company shall
not be prohibited by any law or governmental order or regulation. All necessary
consents, approvals, licenses, permits, orders and authorizations of, or
registrations, declarations and filings with, any governmental or
administrative agency or of any other person with respect to any of the
transactions contemplated under this Agreement to be consummated at the Closing
shall have been duly obtained or made and shall be in full force and effect.

           (f) All instruments and corporate proceedings in connection with the
transactions contemplated by this Agreement to be consummated at the Closing
shall be reasonably satisfactory in form and substance to the Company, and the
Company shall have received counterpart originals, or certified or other copies
of all documents, including without limitation records of corporate or other
proceedings, which it may have reasonably requested in connection therewith.

      6.   *


      7.   Restrictions on Transfer.

      7.1 Contractual Restrictions on Transfer of Warrant. Notwithstanding
anything expressed or implied in this Agreement (including, without limitation,
Section 7.3 hereof) to the contrary, the Purchaser shall not sell, assign,
transfer, pledge, mortgage, hypothecate or otherwise convey, dispose of or
encumber all or any portion of the Warrant or any interest therein to any
Affiliate of the Purchaser or any Prohibited Transferee unless such Affiliate
or such Prohibited Transferee, as the case may be, agrees in writing to be
bound by all of the terms and conditions of this Section 7 and Section 6 hereof
to the same extent as the Purchaser has agreed to be bound or unless the
Company shall consent in writing to any such sale, assignment, transfer,
pledge, mortgage, hypothecation or other conveyance, disposition or
encumbrance. The restrictions on transfer set forth in this Section 7.1 shall
not apply to the sale (other than to an Affiliate of the Purchaser) of all or
any portion of the Warrant in the public market pursuant to a "brokers'
transaction" (as defined under subsection (g) of Rule 144.)

      7.2 Contractual Restrictions on Transfer of Shares. Notwithstanding
anything expressed or implied in this Agreement (including, without limitation,


*  Confidential treatment requested; material has been omitted and filed
   separately with the Commission.



Section 7.3 hereof) to the contrary, the Purchaser shall not sell, assign,
transfer, pledge, mortgage, hypothecate or otherwise convey, dispose of or
encumber any of the Shares or any interest therein to any Affiliate of the
Purchaser or any Prohibited Transferee unless such Affiliate or such Prohibited
Transferee, as the case may be, agrees in writing to be bound by all of the
terms and conditions of this Section 7 and Section 6 hereof to the same extent
as the Purchaser has agreed to be bound or unless the Company shall consent in
writing to any such sale, assignment, transfer, pledge, mortgage, hypothecation
or other conveyance, disposition or encumbrance. The restrictions on transfer
set forth in this Section 7.2 shall not apply to (i) the sale of any Shares to
a Prohibited Transferee pursuant to an underwritten public offering registered
under the Securities Act, (ii) the sale (other than to an Affiliate of the
Purchaser) of any Shares in the public market pursuant to a "brokers'
transaction" (as defined under subsection (g) of Rule 144) and (iii) any hedge
position established by the Purchaser.

      7.3. Securities Laws Restrictions on Transfer. Notwithstanding anything
expressed or implied in this Agreement (including, without limitation, Section
7.1 or Section 7.2 hereof) to the contrary, the Purchaser shall not offer,
sell, assign, transfer, pledge, mortgage, hypothecate or otherwise convey,
dispose of or encumber any of the Securities or any interest therein, except
(i) pursuant to an effective registration statement under the Securities Act,
(ii) pursuant to an available exemption from registration under the Securities
Act and applicable state securities laws and, if requested by the Company, upon
delivery by the Purchaser of an opinion of counsel (satisfactory to the Company
as to such counsel and as to the substance of such opinion) to the effect that
the proposed offer, sale, assignment, transfer, pledge, mortgage, hypothecation
or other conveyance, disposition or encumbrance is exempt from registration
under the Securities Act and applicable state securities laws or (iii) pursuant
to Rule 144.

      7.4 Effect of Violation of Transfer Restrictions; Preventive Measures.
Any offer, sale, assignment, transfer, endorsement, pledge, mortgage,
hypothecation, or other conveyance or disposition of any or all of the
Securities or any interest therein in violation of this Section 7 shall be null
and void. The Company may make a notation on its records or give instructions
to any of its transfer agents in order to implement the restrictions on
transfer set forth in this Section 7. The Company shall not incur any liability
for any delay in recognizing any transfer of any Security if and only for so
long as the Company reasonably believes that any such transfer may have been or
would be in violation of the provisions of the Securities Act, applicable blue
sky laws or this Section 7.




      7.5. Legends.

           (a) To the extent applicable, each certificate or other document
evidencing any of the Common Shares or the Conversion Shares shall be endorsed
with the legend set forth below, and the Purchaser covenants that, except to
the extent such restrictions are waived by the Company, it shall not transfer
the shares represented by any such certificate without complying with the
restrictions on transfer described in this Agreement and the legends endorsed
on such certificate:

      "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
      UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR THE SECURITIES LAWS OF
      ANY STATE AND MAY NOT BE OFFERED, SOLD, TRANSFERRED, ASSIGNED, PLEDGED,
      HYPOTHECATED, MORTGAGED OR OTHERWISE CONVEYED, DISPOSED OF OR ENCUMBERED
      UNLESS THEY ARE SO REGISTERED, UNLESS AN EXEMPTION FROM SUCH REGISTRATION
      IS AVAILABLE AND, IF REQUESTED BY LEUKOSITE, AN OPINION OF COUNSEL
      (SATISFACTORY TO LEUKOSITE AS TO SUCH COUNSEL AND AS TO THE SUBSTANCE OF
      SUCH OPINION) IS DELIVERED TO LEUKOSITE TO THE EFFECT THAT SUCH
      REGISTRATION IS NOT UNDER THE CIRCUMSTANCES REQUIRED OR UNLESS SUCH
      SHARES ARE SOLD PURSUANT TO RULE 144 PROMULGATED UNDER THE SECURITIES ACT
      OF 1933, AS AMENDED. THE SHARES REPRESENTED BY THIS CERTIFICATE ARE
      SUBJECT TO CERTAIN RESTRICTIONS ON TRANSFER SET FORTH IN SECTION 7 OF
      THAT CERTAIN SECURITIES PURCHASE AGREEMENT, DATED DECEMBER 18, 1997, AND
      NO TRANSFER OF SUCH SHARES SHALL BE VALID OR EFFECTIVE IF IT IS NOT
      EFFECTED IN COMPLIANCE WITH ALL OF SUCH RESTRICTIONS ON TRANSFER. COPIES
      OF SUCH SECURITIES PURCHASE AGREEMENT MAY BE OBTAINED AT NO COST BY
      WRITTEN REQUEST MADE BY THE HOLDER OF RECORD OF SUCH SHARES TO THE
      SECRETARY OF LEUKOSITE, INC."

           (b) To the extent applicable, each certificate or other document
evidencing any of the Warrant Shares shall be endorsed with the legend set
forth below, and the Purchaser covenants that, except to the extent such
restrictions are waived by the Company, it shall not transfer the shares
represented by any such certificate without complying with the restrictions on
transfer described in this Agreement and the Warrant and the legends endorsed
on such certificate:

      "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
      UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR THE SECURITIES LAWS OF



      ANY STATE AND MAY NOT BE OFFERED, SOLD, TRANSFERRED, ASSIGNED, PLEDGED,
      HYPOTHECATED, MORTGAGED OR OTHERWISE CONVEYED, DISPOSED OF OR ENCUMBERED
      UNLESS THEY ARE SO REGISTERED, UNLESS AN EXEMPTION FROM SUCH REGISTRATION
      IS AVAILABLE AND, IF REQUESTED BY LEUKOSITE, AN OPINION OF COUNSEL
      (SATISFACTORY TO LEUKOSITE AS TO SUCH COUNSEL AND AS TO THE SUBSTANCE OF
      SUCH OPINION) IS DELIVERED TO LEUKOSITE TO THE EFFECT THAT SUCH
      REGISTRATION IS NOT UNDER THE CIRCUMSTANCES REQUIRED OR UNLESS SUCH
      SHARES ARE SOLD PURSUANT TO RULE 144 PROMULGATED UNDER THE SECURITIES ACT
      OF 1933, AS AMENDED. THE SHARES REPRESENTED BY THIS CERTIFICATE ARE
      SUBJECT TO CERTAIN RESTRICTIONS ON TRANSFER SET FORTH IN SECTION 7 OF
      THAT CERTAIN SECURITIES PURCHASE AGREEMENT DATED DECEMBER 18, 1997 AND IN
      SECTION 9 OF THAT CERTAIN WARRANT DATED DECEMBER 18, 1997, AND NO
      TRANSFER OF SUCH SHARES SHALL BE VALID OR EFFECTIVE IF IT IS NOT EFFECTED
      IN COMPLIANCE WITH ALL OF SUCH RESTRICTIONS ON TRANSFER. COPIES OF SUCH
      SECURITIES PURCHASE AGREEMENT AND WARRANT MAY BE OBTAINED AT NO COST BY
      WRITTEN REQUEST MADE BY THE HOLDER OF RECORD OF SUCH SHARES TO THE
      SECRETARY OF LEUKOSITE, INC."

           (c) The Warrant shall be endorsed with the legend set forth below,
and the Purchaser covenants that, except to the extent such restrictions are
waived by the Company, it shall not transfer the Warrants without complying
with the restrictions on transfer described in this Agreement and the Warrant
and the legend endorsed on the Warrant:

      "NEITHER THIS WARRANT NOR THE SHARES ISSUED OR ISSUABLE UPON EXERCISE OF
      THIS WARRANT HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
      AMENDED, OR THE SECURITIES LAWS OF ANY STATE AND MAY NOT BE OFFERED,
      SOLD, TRANSFERRED, ASSIGNED, PLEDGED, HYPOTHECATED, MORTGAGED OR
      OTHERWISE CONVEYED, DISPOSED OF OR ENCUMBERED UNLESS THEY ARE SO
      REGISTERED, UNLESS AN EXEMPTION FROM SUCH REGISTRATION IS AVAILABLE AND,
      IF REQUESTED BY LEUKOSITE, AN OPINION OF COUNSEL (SATISFACTORY TO
      LEUKOSITE AS TO SUCH COUNSEL AND AS TO THE SUBSTANCE OF SUCH OPINION) IS
      DELIVERED TO LEUKOSITE TO THE EFFECT THAT SUCH REGISTRATION IS NOT UNDER
      THE CIRCUMSTANCES REQUIRED OR UNLESS THIS WARRANT OR SUCH SHARES ARE SOLD
      PURSUANT TO RULE 144 PROMULGATED UNDER THE SECURITIES ACT OF 1933, AS
      AMENDED. THIS WARRANT AND THE SHARES ISSUED OR ISSUABLE UPON EXERCISE OF
      THIS WARRANT ARE SUBJECT TO CERTAIN RESTRICTIONS ON TRANSFER SET FORTH IN



      SECTION 9 OF THIS WARRANT AND IN SECTION 7 OF THAT CERTAIN SECURITIES
      PURCHASE AGREEMENT DATED DECEMBER 18, 1997, AND NO TRANSFER OF THIS
      WARRANT AND/OR SUCH SHARES SHALL BE VALID OR EFFECTIVE IF IT IS NOT
      EFFECTED IN COMPLIANCE WITH ALL OF SUCH RESTRICTIONS ON TRANSFER. COPIES
      OF SUCH SECURITIES PURCHASE AGREEMENT MAY BE OBTAINED AT NO COST BY
      WRITTEN REQUEST MADE BY THE HOLDER OF RECORD OF THIS WARRANT AND/OR SUCH
      SHARES TO THE SECRETARY OF LEUKOSITE, INC."

      8.   Miscellaneous Provisions.

      8.1 Public Statements or Releases. None of the parties to this Agreement
shall make, issue, or release any announcement, whether to the public
generally, or to any of its employees, suppliers, or customers, with respect to
the financial terms of this Agreement or the transactions provided for herein
or with respect to a description of the provisions of Section 6 hereof without
the prior consent of the other party, which shall not be unreasonably withheld
or delayed, provided, that nothing in this Section 8.1 shall prevent any of the
parties hereto from making such public announcements as it may consider
necessary in order to satisfy its legal obligations, but to the extent not
inconsistent with such obligations, it shall provide the other party with an
opportunity to review and comment on any such proposed public announcement
before it is made.

      8.2 Further Assurances. Each party agrees to cooperate fully with the
other party and to execute such further instruments, documents and agreements
and to give such further written assurances, as may be reasonably requested by
the other party to better evidence and reflect the transactions described
herein and contemplated hereby, and to carry into effect the intents and
purposes of this Agreement.

      8.3 Rights Cumulative. Each and all of the various rights, powers and
remedies of the parties shall be considered to be cumulative with and in
addition to any other rights, powers and remedies which such parties may have
at law or in equity in the event of the breach of any of the terms of this
Agreement. The exercise or partial exercise of any right, power or remedy shall
neither constitute the exclusive election thereof nor the waiver of any other
right, power or remedy available to such party.




      8.4 Pronouns. All pronouns or any variation thereof shall be deemed to
refer to the masculine, feminine or neuter, singular or plural, as the identity
of the person, persons, entity or entities may require.

      8.5 Notices. Any notice, demand, request or other communication hereunder
to either party shall be deemed to be sufficient if contained in a written
instrument delivered in person or duly sent by overnight courier service or by
first class, registered or certified mail, postage prepaid, or telecopied with
a confirmation copy by regular, certified or overnight mail, addressed or
telecopied, as the case may be, to such party at the address or telecopier
number, as the case may be, set forth below or such other address or telecopier
number, as the case may be, as may hereafter be designated in writing by the
addressee to the addressor:

           LeukoSite, Inc.
           215 First Street
           Cambridge, MA 02142
           Attention:President (one copy)
                      Chief Financial Officer (another copy)
           Telecopier:  (617) 621-9349

           and a copy to:

           Bingham Dana LLP
           150 Federal Street
           Boston, Massachusetts 02110
           Attention: Justin P. Morreale, Esq.
           Telecopier: (617) 951-8736


           Genentech, Inc.
           1 DNA Way
           South San Francisco, CA  94080
           Attention:  Corporate Secretary
           Telecopier: 650-952-9881

      All of such notices, requests and other communications shall be deemed to
have been received: (i) in the case of personal delivery, on the date of such
delivery; (ii) in the case of overnight courier service, the business day
following the date of receipt by the overnight courier; (iii) in the case of
first class, registered or certified mail, on the earlier of (A) receipt
thereof or (B) the fifth business day after deposit in the mail; and (iv) in
the case of facsimile transmission, when confirmed by facsimile machine report.




      8.6 Captions. The captions and paragraph headings of this Agreement are
solely for the convenience of reference and shall not affect its
interpretation.

      8.7 Severability. Should any part or provision of this Agreement be held
unenforceable or in conflict with the applicable laws or regulations of any
jurisdiction, the invalid or unenforceable part or provisions shall be replaced
with a provision which accomplishes, to the extent possible, the original
business purpose of such part or provision in a valid and enforceable manner,
and the remainder of this Agreement shall remain binding upon the parties
hereto.

      8.8  Governing Law; Injunctive Relief.

      (a) This Agreement shall be governed by and construed in accordance with
the internal and substantive laws of the State of Delaware and without regard
to any conflicts of laws concepts which would apply the substantive law of some
other jurisdiction.

      (b) Each of the parties hereto acknowledges and agrees that damages will
not be an adequate remedy for any material breach or violation of this
Agreement if such material breach or violation would cause immediate and
irreparable harm (an "Irreparable Breach"). Accordingly, in the event of a
threatened or ongoing Irreparable Breach, each party hereto shall be entitled
to seek, in any state or federal court in the Commonwealth of Massachusetts or
the State of California, equitable relief of a kind appropriate in light of the
nature of the ongoing or threatened Irreparable Breach, which relief may
include, without limitation, specific performance or injunctive relief. Such
remedies shall not be the parties' exclusive remedies, but shall be in addition
to all other remedies provided in this Agreement.

      (c) In any action or proceeding brought to enforce any provision of this
Agreement or where any provision hereof is validly asserted as a defense, the
successful party to such action or proceeding shall be entitled to recover
reasonable attorney's fees and disbursements in connection with such action or
proceeding.

      8.9 Waiver. No waiver of any term, provision or condition of this
Agreement, whether by conduct or otherwise, in any one or more instances, shall
be deemed to be, or be construed as, a further or continuing waiver of any such
term, provision or condition or as a waiver of any other term, provision or
condition of this Agreement.

      8.10 Expenses. Each party will bear its own costs and expenses in
connection with the preparation, negotiation and execution of this Agreement.




      8.11  Assignment. The rights and obligations of either party hereto shall
inure to the benefit of and shall be binding upon any successor of the business
of such party (whether by merger or otherwise) and the permitted assigns of
such party. Except to the extent otherwise expressly provided or contemplated
elsewhere in this Agreement, neither party may assign its rights or obligations
under this Agreement or designate another person (i) to perform all or part of
its obligations under this Agreement or (ii) to have all or part of its rights
and benefits under this Agreement, without the prior written consent of the
other party.

      8.12 Survival. The respective representations and warranties given by the
parties hereto, and the other covenants and agreements contained herein, shall
survive the Closing Date and the consummation of the transactions contemplated
herein for a period of two years, without regard to any investigation made by
any party.

      8.13 Entire Agreement. This Agreement constitutes the entire agreement
between the parties hereto respecting the subject matter hereof and supersedes
all prior agreements, negotiations, understandings, representations and
statements respecting the subject matter hereof, whether written or oral.

      8.14 Amendments. No modification, alteration, waiver or change in any of
the terms of this Agreement shall be valid or binding upon the parties hereto
unless made in writing and duly executed by the parties hereto.

      8.15 Hart-Scott-Rodino Compliance. Prior to the Closing, the Purchaser
shall make a determination as to whether the execution and delivery of, or the
performance of the obligations of the Company or the Purchaser under, this
Agreement and the Collaboration Agreement (including, without limitation, the
consummation of the Closing and the issuance of the Common Shares and Warrant
to the Purchaser at the Closing) shall require that filings under the
Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (the "HSR
Act"), or the rules and regulations promulgated thereunder, be made prior to
the Closing by the Company, the Purchaser or any of their respective Affiliates
or ultimate parent entities, if any. If the Purchaser makes a determination
that such filings are required, the Purchaser shall give written notice of such
determination to the Company prior to the Closing.

      8.16 Counterparts. This Agreement may be executed in a number of
counterparts, all of which together shall for all purposes constitute one
Agreement binding on all the parties hereto notwithstanding that all of such
parties have not signed the same counterpart.






      IN WITNESS WHEREOF, the parties hereto have executed this Agreement under
seal as of the day and year first above written.


                                    LEUKOSITE, INC.


                                    By:__________________________
                                       Christopher K. Mirabelli,
                                       President


                                    GENENTECH, INC.


                                    By:__________________________
                                    Name:________________________
                                    Title:_______________________