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                                                                  Exhibit 10.35


                         EXECUTIVE EMPLOYMENT AGREEMENT

         THIS EXECUTIVE EMPLOYMENT AGREEMENT (the "Agreement"), made this 14th
day of October, 1999, is entered into by Millennium Pharmaceuticals, Inc., a
Delaware corporation with its principal place of business at 75 Sidney Street,
Cambridge, MA 02139 (the "Company"), and Christopher K. Mirabelli, residing at 6
Pine Street, Dover, MA 02038 (the "Executive").

         The Company desires to employ the Executive, and the Executive desires
to be employed by the Company. In consideration of the mutual covenants and
promises contained herein, and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged by the parties hereto,
the parties agree as follows:

         1. TERM OF EMPLOYMENT. The Company hereby agrees to employ the
Executive, and the Executive hereby accepts employment with the Company, upon
the terms set forth in this Agreement, for the period commencing on the Closing
Date (as defined in Agreement and Plan of Merger (the "Merger Agreement") among
the Company, ANM, Inc., and Leukosite, Inc., a Delaware corporation) and ending
on the date that is three years after the Closing Date, unless sooner terminated
in accordance with the provisions of Section 4 (such period, the "Employment
Period"). Notwithstanding anything in this Agreement to the contrary, in the
event of the termination of the Merger Agreement for any reason, this Agreement
shall simultaneously terminate and be of no further force or effect, and neither
party shall have any liability to the other.

         2. TITLE; CAPACITY. The Executive shall serve in such position as the
Company or its Board of Directors (the "Board") and the Executive may determine
from time to time. The Executive shall be based at the Company's headquarters in
Cambridge, Massachusetts, or such place or places in the greater Boston,
Massachusetts area as the Board shall determine. The Executive shall be subject
to the supervision of, and shall have such authority as is delegated to the
Executive by, the Board or the Chief Executive Officer of the Company.

         The Executive hereby accepts such employment and agrees to undertake
such duties and responsibilities as the Board or its designee shall from time to
time reasonably assign to the Executive. The Executive agrees to devote his
entire business time, attention and energies to the business and interests of
the Company during the Employment Period. The Executive agrees to abide by the
rules, regulations, instructions, personnel practices and policies of the
Company and any changes therein which may be adopted from time to time by the
Company.

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         3. COMPENSATION AND BENEFITS.

                  3.1 SALARY. The Company shall pay the Executive, in periodic
installments in accordance with the Company's customary payroll practices,
salary at the annual rate of $287,179 or such greater amount as the Board or any
compensation committee of the Board (the "Compensation Committee") may from time
to time determine. The Executive's salary will be reviewed by the Board or the
Compensation Committee on an annual basis.

                  3.2 BONUS. The Employee shall be eligible to participate in
such annual cash bonus programs as may be established from time to time for its
senior executives, on terms similar to those applicable to other senior
executives.

                  3.3 FRINGE BENEFITS. The Executive shall be entitled to
participate in all benefit programs, if any, that the Company establishes and
makes available to its employees or senior executives, to the extent that
Executive's position, tenure, salary, age, health and other qualifications make
him eligible to participate.

                  3.4 REIMBURSEMENT OF EXPENSES. The Company shall reimburse the
Executive for all reasonable travel, entertainment and other expenses incurred
or paid by the Executive in connection with, or related to, the performance of
his duties, responsibilities or services under this Agreement, in accordance
with policies and procedures, and subject to limitations, adopted by the Company
from time to time.

                  3.5 WITHHOLDING. All salary, bonus and other compensation
payable to the Executive shall be subject to applicable withholding taxes.

                  3.6 CERTAIN COMPANY OPTIONS. Twenty-five percent of any
options to purchase Company common stock granted to the Executive by the Company
between the Closing Date and the first anniversary of the Closing Date shall
become exercisable on the first anniversary of the Closing Date if (i) the
Executive has completed at least one consecutive year of employment with the
Company after the Closing Date or (ii) the Executive has completed at least six
consecutive months of employment with the Company after the Closing Date and the
Executive has complied with his obligations under Section 5.1(b)(ii) and
Sections 6 and 7 during the six-month period commencing on the date of such
termination.

                  3.7 CERTAIN TAX GROSS-UP PAYMENTS.

                  (a) The Company shall, within 30 days after each date on which
the Executive becomes entitled to receive (whether or not then due) a Contingent
Compensation Payment, determine in good faith and deliver to the Executive its
calculations (the "Gross-Up Payment Calculations"), with reasonable detail
regarding the basis for its determinations, as to (i) which of the payments or
benefits due to the

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Executive (under this Agreement or otherwise) constitute
Contingent Compensation Payments, (ii) the amount, if any, of Excise Tax payable
by the Executive relating to such Contingent Compensation Payment and (iii) the
amount of the Gross-Up Payment (as defined below) due to the Executive with
respect to such Contingent Compensation Payment. The Executive shall have a
reasonable opportunity to review and discuss with the Company the Gross-Up
Payment Calculations, and in the event the Executive does not approve of such
calculations, the parties shall cooperate in good faith to reach agreement. If
such disagreement is not resolved within a 60-day period following the delivery
by the Company to the Executive of its Gross-Up Payment Calculations, such
dispute shall, at the request of either party, be settled exclusively and
expeditiously by the Company's independent auditors (which must be a "Big Five"
firm). Judgment may be entered on the independent auditor's determination in any
court having jurisdiction. On the later to occur of (A) 90 days after the due
date of each Contingent Compensation Payment to the Executive (or, if earlier,
the due date for payment of the Excise Tax relating to such Contingent
Compensation Payment), or (B) five days following the date the Company and the
Executive reach agreement (or the date of the arbitrator's determination) on the
amount of the Gross-Up Payment with respect to such Contingent Compensation
Payment, the Company shall pay to the Executive, in cash, the Gross-Up Payment
with respect to such Contingent Compensation Payment. Notwithstanding the
foregoing, as a condition to the obligation of the Company to pay any Gross-Up
Payments, the Executive shall be required to take such actions without
substantial incremental expense to the Executive which the Company may suggest
and as to which the Executive may agree (not to be unreasonably withheld) as
being appropriate to minimize or avoid any liability for Excise Taxes on any of
the Contingent Compensation Payments to be made to the Executive. Without
limiting the foregoing, the Company and the Executive shall discuss in good
faith the following actions by the Executive: (i) exercising during 1999 all
non-statutory stock options currently held by the Executive which will be vested
on or prior to December 31, 1999 (or such lesser number of options as is
determined by the Company to be necessary to eliminate any liability for the
Excise Tax on any such Contingent Compensation Payment; and (ii) using other
reasonable efforts to increase his 1999 taxable income so as to minimize or
avoid the application of the Excise Tax on the Contingent Compensation Payments,
including, at the request of the Company, requesting Leukosite, Inc. to
accelerate such additional number of unvested non-statutory options prior to
December 31, 1999 as the Company may request and exercising during 1999 any such
options so accelerated.

                  (b) For purposes of this Section 3.7, the following terms
shall have the following respective meanings:

                           (i) "Contingent Compensation Payment" shall mean any
payment to the Executive, regardless of whether payable under this Agreement,
which



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relative to Leukosite, Inc. is or may be a "parachute payment" within the
meaning of Section 280G(b)(2)(A) of the Code (Determined without reference to
the threshold of Section 280G(b)(2)(A)(ii) of the Code).

                           (ii) "Gross-Up Payment" shall mean an amount equal to
the sum of (i) the amount of the Excise Tax payable with respect to a Contingent
Compensation Payment and (ii) the amount necessary to pay all additional taxes
imposed on (or economically borne by) the Executive (including the Excise Taxes,
state and federal income and employment taxes and all applicable withholding
taxes) attributable to the receipt of such Gross-Up Payment. For purposes of the
preceding sentence, all taxes attributable to the receipt of the Gross-Up
Payment shall be computed assuming the application of the maximum tax rates
provided by law.

                           (iii) "Excise Tax" shall mean the excise tax payable
pursuant to Section 4999 of the Internal Revenue Code of 1986, as amended (the
"Code").

         4. TERMINATION OF EMPLOYMENT PERIOD. The employment of the Executive by
the Company pursuant to this Agreement (but not any consulting services that the
Executive may be required to render pursuant to Section 5.1(b)(ii) hereof) shall
terminate upon the occurrence of any of the following:

                  4.1 At the election of the Company, for Cause (as defined
below), immediately upon written notice by the Company to the Executive (unless
such Cause is capable of being cured, in which case 15 days following the date
of such written notice if such Cause has not been cured by such time), which
notice shall identify the Cause upon which the termination is based.
Notwithstanding the foregoing, the Company shall not terminate the Executive's
employment for Cause unless and until there shall have been delivered to him a
copy of a resolution duly adopted by the affirmative vote of not less than a
majority of the entire Board of Directors at a meeting of the Board of Directors
called and held for such purpose (after reasonable notice to the Executive and
an opportunity for him, together with his counsel, to be heard before the Board
of Directors), finding that in the good faith opinion of the Board of Directors
the Executive was guilty of the conduct enumerated in any of clauses (i) through
(iii) below under the definition of Cause and specifying the particulars thereof
in detail. For the purposes of this Section 4.1, "Cause" shall mean (i) the
Executive has failed to perform his reasonably assigned duties for the Company
and has failed to remedy such failure within 10 days following written notice
from the Company to the Executive notifying him of such failure, (ii) the
Executive has engaged in dishonesty, gross negligence or willful misconduct in a
manner which is detrimental to the Company, or (iii) the conviction of the
Executive of, or the entry of a pleading of guilty



                                       4
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or nolo contendere by the Executive to, any crime involving moral turpitude or
any felony;

                  4.2 Upon the death or disability of the Executive. As used in
this Agreement, the term "disability" shall mean the inability of the Executive,
due to a physical or mental disability, for a period of 90 days, whether or not
consecutive, during the Employment Period to perform the services contemplated
under this Agreement, with or without reasonable accommodation as that term is
defined under state or federal law. A determination of disability shall be made
by a physician satisfactory to both the Executive and the Company, PROVIDED THAT
if the Executive and the Company do not agree on a physician, the Executive and
the Company shall each select a physician and these two together shall select a
third physician, whose determination as to disability shall be binding on all
parties;

                  4.3 At the election of the Company, without Cause, following
30 days' prior written notice by the Company to the Executive; or

                  4.4 At the election of the Executive, following 30 days' prior
written notice by the Executive to the Company.

         5. EFFECT OF TERMINATION.

                  5.1 PAYMENTS UPON TERMINATION.

                  (a) In the event the Executive's employment is terminated
pursuant to Section 4, the Company shall pay to the Executive the compensation
and benefits otherwise payable to him under Section 3 through the last day of
his actual employment by the Company.

                  (b) In the event the Executive's employment is terminated
during the Employment Period (i) by the Company, without Cause, (ii) by the
Executive after the Executive has completed at least six consecutive months of
employment with the Company after the Closing Date, (iii) by the Executive for
Good Reason (as defined below) prior to the completion by the Executive of at
least six consecutive months of employment with the Company after the Closing
Date, or (iv) as a result of the Executive's death or disability:

                           (i) the Company shall continue to pay to the
Executive his salary pursuant to Section 3.1 above as in effect on the date of
termination and continue to provide to the Executive the other benefits owed to
him under Section 3.3 (to the extent such benefits can be provided to
non-employees) until, in each such case, the date 18 months after the date of
termination. In addition, the Company shall pay to the Executive in a lump sum
upon the date of termination an amount equal to six months' salary pursuant to
Section 3.1 above; and


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                           (ii) as partial consideration for the Company's
agreements in Section 5.1(b)(i), the Executive shall, at the Company's
request, provide the Company with two days of consulting services per month
during the six-month period commencing on the date of such termination of
employment. The Executive shall perform such consulting services at such
reasonable times and places as shall be designated by the Company upon
reasonable prior notice to the Executive. The Company shall promptly reimburse
the Executive for all reasonable out-of-pocket expenses incurred by the
Executive in performing such consulting services. For purposes of this
Agreement, the term "Good Reason" shall mean any of the following:

                           (A) the assignment to the Executive of any duties
                  inconsistent with the Executive's status as a senior executive
                  officer of the Company;

                           (B) a reduction by the Company in the Executive's
                  annual base salary as in effect on the date hereof or as the
                  same may be increased from time to time;

                           (C) the Company's requiring the Executive to be based
                  anywhere other than the Boston Metropolitan Area;

                           (D) the failure by the Company, without the
                  Executive's consent, to pay to the Executive any portion of
                  the Executive's compensation within fourteen days of the date
                  such compensation is due; or

                           (E) any purported termination of the Executive's
                  employment which is not effected pursuant to, and in
                  accordance with, the provisions of Section 4 hereof.

                  5.2 NO DUTY TO MITIGATE DAMAGES. The Executive shall not be
required to mitigate the amount of any payment or benefit to which he may be
entitled pursuant to this Agreement following the termination of the Executive's
employment hereunder, whether by seeking other employment or otherwise, and the
amount of any such payment or benefit shall not be reduced, in whole or in part,
by the amount of any compensation or benefit earned by the Executive from any
endeavor (including other employment) following such termination.


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                  5.3 DEATH OR INCOMPETENCE. In the event that the Executive
shall die or shall become incompetent, any and all payments that the Company
shall be required to make to the Executive pursuant to this Agreement shall be
made to the executors, administrators, legal representatives or heirs, as the
case may be, of the Executive.

                  5.4 RESIGNATION OF BOARD SEAT. The Executive agrees to resign
as a member of the Board of Directors of the Company immediately upon, and
effective with, the termination or cessation of his employment by the Company
for any reason, and the Company shall have no obligation to make any payments to
the Executive under this Section 5 until the Executive shall have so resigned as
a member of the Board (and any payments otherwise due during the period the
Executive remains as a member of the Board of Directors shall be forfeited).

                  5.5 SURVIVAL. The provisions of Sections 5, 6, 7 and 9 shall
survive the termination of this Agreement.

         6.       NON-SOLICITATION.

                  6.1 RESTRICTED ACTIVITIES. While the Executive is employed by
the Company and for a period of two years after the termination or cessation of
such employment for any reason, the Executive will not directly or indirectly,
either alone or in association with others, (i) solicit, or permit any
organization directly or indirectly controlled by the Executive to solicit, any
Executive of the Company to leave the employ of the Company, or (ii) solicit for
employment, hire or engage as an independent contractor, or permit any
organization directly or indirectly controlled by the Executive to solicit for
employment, hire or engage as an independent contractor, any person who was
employed by the Company at any time during the term of the Employee's employment
with the Company or service as a consultant to the Company pursuant to Section
5.1(b)(ii); PROVIDED, that this clause (ii) shall not apply to any individual
whose employment with the Company has been terminated for a period of six months
or longer.

                  6.2 EXTENSION. If the Executive violates the provisions of
Section 6.1, the Executive shall continue to be bound by the restrictions set
forth in Section 6.1 until a period of two years has expired without any
violation of such provisions.

                  6.3 INTERPRETATION. If any restriction set forth in this
Section 6 is found by any court of competent jurisdiction to be unenforceable
because it extends for too long a period of time or over too great a range of
activities or in too broad a geographic area, it shall be interpreted to extend
only over the maximum period of time, range of activities or geographic area as
to which it may be enforceable. For purposes of this Section 6 and Section 7,
the Company shall include all subsidiaries and other affiliates of the Company.


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                  6.4 EQUITABLE REMEDIES. The restrictions contained in this
Section 6 are necessary for the protection of the business and goodwill of the
Company and are considered by the Executive to be reasonable for such purpose.
The Executive agrees that any breach of this Section 6 is likely to cause the
Company substantial and irrevocable damage and therefore, in the event of any
such breach, the Executive agrees that the Company, in addition to such other
remedies which may be available, shall be entitled to specific performance and
other injunctive relief.

         7. PROPRIETARY INFORMATION AND DEVELOPMENTS.

                  7.1 PROPRIETARY INFORMATION.

                  (a) The Executive agrees that all information, whether or not
in writing, of a private, secret or confidential nature concerning the Company's
business, business relationships or financial affairs (collectively,
"Proprietary Information") is and shall be the exclusive property of the
Company. By way of illustration, but not limitation, Proprietary Information may
include inventions, products, processes, methods, techniques, formulas,
compositions, compounds, projects, developments, plans, research data, clinical
data, financial data, personnel data, computer programs, customer and supplier
lists, and contacts at or knowledge of customers or prospective customers of the
Company. The Executive will not disclose any Proprietary Information to any
person or entity other than employees of the Company or use the same for any
purposes (other than in the performance of his duties as an Executive of the
Company) without written approval by an officer of the Company, either during or
after his employment with the Company, unless and until such Proprietary
Information has become public knowledge without fault by the Executive.

                  (b) The Executive agrees that all files, letters, memoranda,
reports, records, data, sketches, drawings, laboratory notebooks, program
listings, or other written, photographic, or other tangible material containing
Proprietary Information, whether created by the Executive or others, which shall
come into his custody or possession, shall be and are the exclusive property of
the Company to be used by the Executive only in the performance of his duties
for the Company. All such materials or copies thereof and all tangible property
of the Company in the custody or possession of the Executive shall be delivered
to the Company, upon the earlier of (i) a request by the Company or (ii)
termination of his employment. After such delivery, the Executive shall not
retain any such materials or copies thereof or any such tangible property.

                  (c) The Executive agrees that his obligation not to disclose
or to use information and materials of the types set forth in paragraphs (a) and
(b) above, and his obligation to return materials and tangible property, set
forth in paragraph (b) above, also extends to such types of information,
materials and tangible property of customers


                                       8
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of the Company or suppliers to the Company or other third parties who may have
disclosed or entrusted the same to the Company or to the Executive.

                  7.2 DEVELOPMENTS.

                  (a) The Executive will make full and prompt disclosure to the
Company of all inventions, improvements, discoveries, methods, developments,
software, and works of authorship, whether patentable or not, which are created,
made, conceived or reduced to practice by him or under his direction or jointly
with others during his employment by the Company or service as a consultant to
the Company pursuant to Section 5.1(b)(ii), whether or not during normal working
hours or on the premises of the Company (all of which are collectively referred
to in this Agreement as "Developments").

                  (b) The Executive agrees to assign and does hereby assign to
the Company (or any person or entity designated by the Company) all his right,
title and interest in and to all Developments and all related patents, patent
applications, copyrights and copyright applications. However, this paragraph (b)
shall not apply to Developments which do not relate to the present or planned
business or research and development of the Company and which are made and
conceived by the Executive not during normal working hours, not on the Company's
premises and not using the Company's tools, devices, equipment or Proprietary
Information. The Executive understands that, to the extent this Agreement shall
be construed in accordance with the laws of any state which precludes a
requirement in an Executive agreement to assign certain classes of inventions
made by an employee, this paragraph (b) shall be interpreted not to apply to any
invention which a court rules and/or the Company agrees falls within such
classes. The Executive also hereby waives all claims to moral rights in any
Developments.

                  (c) The Executive agrees to cooperate fully with the Company,
both during and after his employment with the Company, with respect to the
procurement, maintenance and enforcement of copyrights, patents and other
intellectual property rights (both in the United States and foreign countries)
relating to Developments. The Executive shall sign all papers, including,
without limitation, copyright applications, patent applications, declarations,
oaths, formal assignments, assignments of priority rights, and powers of
attorney, which the Company may deem necessary or desirable in order to protect
its rights and interests in any Development. The Executive further agrees that
if the Company is unable, after reasonable effort, to secure the signature of
the Executive on any such papers, any executive officer of the Company shall be
entitled to execute any such papers as the agent and the attorney-in-fact of the
Employee, and the Executive hereby irrevocably designates and appoints each
executive officer of the Company as his agent and attorney-in-fact to execute
any such



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papers on his behalf, and to take any and all actions as the Company may deem
necessary or desirable in order to protect its rights and interests in any
Development, under the conditions described in this sentence.

                  7.3. UNITED STATES GOVERNMENT OBLIGATIONS. The Executive
acknowledges that the Company from time to time may have agreements with other
parties or with the United States Government, or agencies thereof, which impose
obligations or restrictions on the Company regarding inventions made during the
course of work under such agreements or regarding the confidential nature of
such work. The Executive agrees to be bound by all such obligations and
restrictions which are made known to the Executive and to take all appropriate
action necessary to discharge the obligations of the Company under such
agreements.

                  7.4 EQUITABLE REMEDIES. The restrictions contained in this
Section 7 are necessary for the protection of the business and goodwill of the
Company and are considered by the Executive to be reasonable for such purpose.
The Executive agrees that any breach of this Section 7 is likely to cause the
Company substantial and irrevocable damage which is difficult to measure.
Therefore, in the event of any such breach, the Executive agrees that the
Company, in addition to such other remedies which may be available, shall have
the right to obtain an injunction from a court restraining such a breach or
threatened breach and the right to specific performance of the provisions of
this Section 7.

         8. OTHER AGREEMENTS. The Executive represents that his performance of
all the terms of this Agreement and the performance of his duties as an
Executive of the Company do not and will not breach any agreement with any prior
employer or other party to which the Executive is a party (including without
limitation any nondisclosure or non-competition agreement).

         9. MISCELLANEOUS.

                  9.1 NOTICES. Any notice delivered under this Agreement shall
be deemed duly delivered four business days after it is sent by registered or
certified mail, return receipt requested, postage prepaid, or one business day
after it is sent for next-business day delivery via a reputable nationwide
overnight courier service, in each case to the address of the recipient set
forth on the signature page hereto. Either party may change the address to which
notices are to be delivered by giving notice of such change to the other party
in the manner set forth in this Section 9.1.

                  9.2 PRONOUNS. Whenever the context may require, any pronouns
used in this Agreement shall include the corresponding masculine, feminine or
neuter forms, and the singular forms of nouns and pronouns shall include the
plural, and vice versa.

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                  9.3 ENTIRE AGREEMENT. This Agreement constitutes the entire
agreement between the parties and supersedes all prior agreements and
understandings, whether written or oral, relating to the subject matter of this
Agreement.

                  9.4 AMENDMENT. This Agreement may be amended or modified only
by a written instrument executed by both the Company and the Executive.

                  9.5 GOVERNING LAW. This Agreement shall be governed by and
construed in accordance with the laws of the Commonwealth of Massachusetts.

                  9.6 SUCCESSORS AND ASSIGNS. This Agreement shall be binding
upon and inure to the benefit of both parties and their respective successors
and assigns, including any corporation with which or into which the Company may
be merged or which may succeed to its assets or business, provided, however,
that the obligations of the Executive are personal and shall not be delegated by
him. The Company shall require any successor (whether direct or indirect, by
purchase, merger, consolidation or otherwise) to all or substantially all of the
business and/or assets of the Company to expressly assume and agree to perform
this Agreement in the same manner and to the same extent that the Company is
required to perform it.

                  9.7 WAIVERS. No delay or omission by the Company in exercising
any right under this Agreement shall operate as a waiver of that or any other
right. A waiver or consent given by the Company on any one occasion shall be
effective only in that instance and shall not be construed as a bar or waiver of
any right on any other occasion.

                  9.8 CAPTIONS. The captions of the sections of this Agreement
are for convenience of reference only and in no way define, limit or affect the
scope or substance of any section of this Agreement.

                  9.9 SEVERABILITY. In case any provision of this Agreement
shall be invalid, illegal or otherwise unenforceable, the validity, legality and
enforceability of the remaining provisions shall in no way be affected or
impaired thereby.

                  9.10 INDEMNIFICATION. The Company shall indemnify and hold
harmless the Executive against all loss, cost, liability and expense arising out
of or relating to the Executive's service to the Company and all of its
subsidiaries and affiliates, whether as an officer, director, employee,
fiduciary of any employee benefit plan or otherwise, to the fullest extent
permitted by law and to the fullest extent provided in the Certificate of
Incorporation and By-Laws of the Company on the date hereof.

                  9.11 SETTLEMENT OF DISPUTES; ARBITRATION. Any dispute or
controversy arising under or in connection with this Agreement shall be settled
exclusively by


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arbitration in Boston, Massachusetts, in accordance with the rules of the
American Arbitration Association then in effect. Judgment may be entered on the
arbitrator' s award in any court having jurisdiction.

                  9.12 ATTORNEYS' FEES. The non-prevailing party shall pay all
of the reasonable legal fees and expenses of the prevailing party's counsel in
connection with any contest or dispute regarding the nature of any termination
of the Executive's employment or in seeking to obtain or enforce any right or
benefit provided by this Agreement.

     THE EXECUTIVE ACKNOWLEDGES THAT HE HAS CAREFULLY READ THIS AGREEMENT AND
UNDERSTANDS AND AGREES TO ALL OF THE PROVISIONS IN THIS AGREEMENT.

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         IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the day and year set forth above.

                                MILLENNIUM PHARMACEUTICALS, INC.

                                By: /s/ MARK LEVIN
                                   ----------------------------------

                                Title:_______________________________

                                Address:   75 Sidney Street
                                           Cambridge, MA  02139

                                EXECUTIVE

                                /s/ CHRISTOPHER K. MIRABELLI
                                ----------------------------------
                                Christopher K. Mirabelli

                                Address:   ____________________________

                                           ____________________________

                                           ____________________________


<PAGE>



                                 FIRST AMENDMENT

                                       TO

                         EXECUTIVE EMPLOYMENT AGREEMENT

         THIS FIRST AMENDMENT TO EXECUTIVE EMPLOYMENT AGREEMENT (this
"Amendment"), made this 21st day of December, 1999, is entered into by and
between Millennium Pharmaceuticals, Inc., a Delaware corporation with its
principal place of business at 75 Sidney Street, Cambridge, MA 02139 (the
"Company"), and Christopher K. Mirabelli, residing at 6 Pine Street, Dover, MA
02038 (the "Executive") and amends that certain Executive Employment Agreement,
dated as of October 14, 1999 (the "Original Agreement"), by and between the
Company and the Executive. Capitalized terms used herein without definition
shall have the meaning for such terms as set forth in the Original Agreement.

         WHEREAS, the Company and the Executive previously entered into the
Original Agreement with respect to prospective employment arrangements between
the Executive and the Company;

         WHEREAS, according to the terms of the Original Agreement, the
Executive agreed to discuss in good faith with the Company the following actions
in connection with minimizing or avoiding liability for Excise Taxes on the
Contingent Compensation Payments to be made to the Executive: (i) exercising
during 1999 all non-statutory stock options currently held by the Executive
which will be vested on or prior to December 31, 1999; and (ii) using other
reasonable efforts to increase the Executive's taxable income, including the
acceleration of additional unvested non-statutory options prior to December 31,
1999 and exercising any such options so accelerated;

         WHEREAS, the consummation of the proposed merger between LeukoSite,
Inc. ("LeukoSite") and ANM, Inc., a wholly-owned subsidiary of the Company,
which is to occur (subject to LeukoSite stockholder approval) on or around
December 22, 1999 renders obsolete the utility of the actions set forth in the
preceding paragraph; and

         WHEREAS, the Executive is nevertheless willing to undertake certain
other actions to minimize or avoid liability for Excise Taxes on the Contingent
Compensation Payments to be made to the Executive for which the Company would
otherwise be responsible under the Original Agreement;

         NOW, THEREFORE, in consideration of the premises contained herein and
for other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the Executive and the Company hereby agree as follows:

         1. The last sentence of Section 3.7(a) is hereby deleted in its
entirety and replaced with the following sentence:


<PAGE>
                                       2

                  "Without limiting the foregoing, the Company and the
                  Executive, after having discussed matters in good faith,
                  hereby agree that the Executive shall (i) consent to the
                  amendment of Section 16 of the LeukoSite 1993 Stock Option
                  Plan, as amended, in or substantially in the form attached
                  hereto (as amended, the "Plan") and (ii) waive full and
                  immediate acceleration rights as to some or all of his Excess
                  ISO Shares (as defined in the Plan) if the Executive is a
                  Restricted ISO Optionee (as defined in the Plan), which Excess
                  ISO Shares shall become exercisable as provided for in the
                  Plan."

         2.       Section 9.10 is hereby amended and restated in its entirety as
                  follows:

                  "9.10 INDEMNIFICATION. The Company shall indemnify and hold
                  harmless the Executive against: (a) all loss, cost, liability
                  and expense arising out of or relating to the Executive's
                  service to the Company and all of its subsidiaries and
                  affiliates, whether as an officer, director, employee,
                  fiduciary of any employee benefit plan or otherwise, to the
                  fullest extent permitted by law and to the fullest extent
                  provided in the Certificate of Incorporation and By-Laws of
                  the Company on the date hereof; and (b) any Excise Taxes (on
                  an after-tax basis, including any Excise Tax and state and
                  federal income and employment taxes, computed on a basis
                  comparable to a Gross-Up Payment, and all interest, additions
                  and penalties thereon) arising out of the Executive's
                  entitlement to or receipt of Contingent Compensation Payments
                  as the same may be finally determined; provided, however, that
                  in the event of either clause (a) or (b), the Employee shall
                  have no right to settle any claims, or make any payments, to
                  any third parties (including, without limitation, the Internal
                  Revenue Service) for which the Company is liable hereunder
                  without the prior written consent of the Company (which
                  consent shall not be unreasonably withheld, conditioned or
                  delayed), and the Company shall have the right to control any
                  proceedings brought by or against any third party to the
                  extent related to matters with respect to which the Company
                  would have liability hereunder."

         3.       Except as expressly provided in this Amendment, all of the
terms and provisions of the Original Agreement shall remain in full force and
effect and all references to the Original Agreement shall hereinafter be deemed
to be references to the Original Agreement as amended by this Amendment.


<PAGE>
                                       3

         4.       This Amendment may be executed in counterparts and shall be
governed by and construed in accordance with the laws of the Commonwealth of
Massachusetts.

                     [REST OF PAGE LEFT INTENTIONALLY BLANK]


<PAGE>
                                       4

         IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the day and year set forth above.

                                            MILLENNIUM PHARMACEUTICALS, INC.

                                            By:  /s/ JOHN B. DOUGLAS III
                                               -------------------------------


                                            Name:

                                            Title:

                                            EXECUTIVE

                                                  /s/ CHRISTOPHER K. MIRABELLI
                                            ----------------------------------
                                                Christopher K. Mirabelli