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                                                                   EXHIBIT 10.26

                              EMPLOYMENT AGREEMENT
                              --------------------

     THIS EMPLOYMENT AGREEMENT (the "Agreement"), made this 1st day of April, is
entered into by The Medicines Company, a Delaware corporation with its principal
place of business at One Cambridge Center, Cambridge, Massachusetts 02142 (the
"Company"), and Thomas Quinn, residing at 325 Independence Drive, Orchard Park,
New York 14127 (the "Employee").

     The Company desires to employ the Employee, and the Employee 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 Employee,
and the Employee hereby accepts employment with the Company, upon the terms set
forth in this Agreement, for the period commencing on April 1, 2000 (the
"Commencement Date") and ending on April 1, 2001 (such period, as it may be
renewed as provided in the following sentence, the "Employment Period"), unless
sooner terminated in accordance with the provisions of Section 4. The Employment
Period shall automatically be renewed for successive one (1) year periods unless
either the Employee or the Company provide written notice of non-renewal to the
other party at least ninety (90) days prior to the expiration of the then
current term.

     2.   TITLE; CAPACITY. The Employee shall serve as Vice President or in such
other position as the Company or its Board of Directors (the "Board") may
determine from time to time. The Employee shall be based at the offices of Stack
Pharmaceuticals, Inc., 5 Sylvan Way, Parsippany, New Jersey, unless otherwise
agreed by the parties. The


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<PAGE>   2

Employee shall be subject to the supervision of, and shall have such authority
as is delegated to him by David Stack, Senior Vice President of the Company (or
such officer of the Company as may be designated by the Board).

The Employee hereby accepts such employment and agrees to undertake the duties
and responsibilities inherent in such position and/or such other duties and
responsibilities as David Stack (or the Board or its designee) shall from time
to time reasonably assign to him. The Employee agrees to devote an average of 24
hours per week to the business and interests of the Company during the
Employment Period. The Company and the Employee generally anticipate that such
hours shall be provided in the form of three business days per week; provided
that (i) the Company and the Employee shall agree upon a work plan on a monthly
basis setting forth the parties' expectations as to when the Employee shall
provide his services to the Company during the forthcoming month and (ii) the
Employee shall have the right to fluctuate his working hours from week to week
as long as the Employee works a total of 288 hours during each three-month
period during the Employment Period. The Employee shall provide the Company on a
monthly basis with a written report setting forth the number of hours/days
worked by the Employee in the previous month. The Employee 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 except as provided in these agreements. The Employee acknowledges
receipt of copies of all such rules and policies committed to writing as of the
date of this Agreement.

     3.   COMPENSATION AND BENEFITS.

          3.1  SALARY. The Company shall pay the Employee, in semi-monthly
installments, an annual base salary of $120,000 for the one-year period
commencing on


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the Commencement Date. Such salary shall be subject to adjustment thereafter as
determined by the Board, but shall not be reduced below the amount set forth
above without the Employee's consent.

          3.2  BONUS. The Employee shall be eligible to receive a bonus of up to
30% of his base salary upon the achievement of annual objectives to be approved
by the CEO of the Company after discussion with the Employee. The Board shall
review the Employee's performance and determine the amount of the bonus, if any,
to be paid to the Employee.

          3.3  REIMBURSEMENT OF EXPENSES. The Company shall reimburse the
Employee for all reasonable travel, entertainment and other expenses incurred or
paid by the Employee in connection with, or related to, the performance of his
duties, responsibilities or services under this Agreement, upon presentation by
the Employee of documentation, expense statements, vouchers and/or such other
supporting information as the Company may request.

          3.4  FRINGE BENEFITS. The Employee shall be entitled to participate in
all other bonus and stock incentive programs that the Company establishes and
makes available to its other employees at the same level as the Employee to the
extent that Employee's position, tenure, salary, age, health and other
qualifications make him eligible to participate. The Employee shall be entitled
to 12 days paid vacation per year, to be taken at such times as may be approved
by David Stack (or the Board or its designee).

     4.   EMPLOYMENT TERMINATION. The employment of the Employee by the Company
pursuant to this Agreement shall terminate upon the occurrence of any of the
following:


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          4.1  EXPIRATION OF EMPLOYMENT PERIOD. Expiration of the Employment
Period in accordance with Section 1.

          4.2  TERMINATION FOR CAUSE. At the election of the Company,
immediately upon written notice by the Company to the Employee, for "cause" as
determined by the Board. For purposes of this Section 4.2, "cause" for
termination shall be deemed to exist only if any of the following shall have
occurred:

     (a)  the Employee's conviction of any crime (whether or not involving the
Company) which constitutes a felony in the jurisdiction involved (other than
unintentional motor vehicle felonies);

     (b)  any act of theft, fraud, misappropriation of funds or embezzlement by
the Employee, in connection with his work with the Company, or any other act or
acts of dishonesty on the part of the Employee resulting or intended to result
directly or indirectly in personal gain or enrichment of the Employee at the
expense of the Company;

     (c)  the Employee's failure to perform in all material respects the
services required to be performed pursuant to Section 2 of this Agreement,
PROVIDED THAT if such failure is capable of being corrected, such failure
continues uncorrected for a period of thirty (30) days after the Employee shall
have received written notice from the Company stating with reasonably
specificity the nature of such failure;

     (d)  the Employee's breach of Sections 6, 7 or 8 of this Agreement;

     (e)  the Employee's excessive use of alcohol and/or drugs which is judged
by the CEO and the Board to materially interfere with the performance of his
duties; or

     (f)  any misconduct by the Employee which in the reasonable judgment of the
CEO and the Board would jeopardize the success of the Company.


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          4.3  DEATH OR DISABILITY. Thirty (30) days after the death or
disability of the Employee. As used in this Agreement, the term "disability"
shall mean the inability of the Employee, due to a physical or mental
disability, for a period of ninety (90) days, whether or not consecutive, during
any 360-day period to perform the services contemplated under this Agreement. A
determination of disability shall be made by a physician satisfactory to both
the Employee and the Company, PROVIDED THAT if the Employee and the Company do
not agree on a physician, the Employee 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.

          4.4  VOLUNTARY TERMINATION. At the election of either party, upon
written notice of termination given at least ninety (90) days prior to the
effective date of termination.

          4.5  VOLUNTARY TERMINATION FOR "GOOD REASON." At the election of the
Employee, upon at least 30 days prior written notice to the Company, for "Good
Reason," which shall be deemed to exist only if:

     (i)  if the Company fails to comply in any material respect with the
provisions of Section 3, other than an isolated, insubstantial and inadvertent
failure which is remedied by the Company promptly after receipt of notice
thereof given by the Employee;

     (ii) the Company shall require the Employee to be based at a location that
is more than 50 miles from Parsippany, New Jersey

          4.6  ANGIOMAX TERMINATION. At any time after September 1, 2000, at the
election of either party, upon written notice of termination given at least
thirty (30) days prior to the effective date of termination, if the Company has
not received written


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notice of FDA approval of, or an approvable letter from the FDA (which does not
require additional clinical trials on the part of the Company) with respect to,
the Company's Angiomax product currently being reviewed by the FDA.

     5.   EFFECT OF TERMINATION.

          5.1  TERMINATION FOR CAUSE OR AT ELECTION OF EMPLOYEE. In the event
the Employee's employment is terminated for cause pursuant to Section 4.2, at
the election of the Employee pursuant to Section 4.4 or 4.5(iii), or at the
election of either party pursuant to Section 4.6, the Company shall pay to the
Employee all sums otherwise payable to him under Section 3 through the last day
of his actual employment by the Company.

          5.2  TERMINATION FOR DEATH OR DISABILITY. If the Employee's employment
is terminated by death or because of disability pursuant to Section 4.3, the
Company shall pay to the estate of the Employee or to the Employee, as the case
may be, all sums which would otherwise be payable to the Employee under Section
3 up to the end of the month in which the termination of his employment because
of death or disability occurs.

          5.3  TERMINATION FOR GOOD REASON OR AT ELECTION OF COMPANY. In the
event that Employee's employment is terminated by the Employee for "Good Reason"
pursuant to Section 4.5(i) or (ii), or at the election of the Company pursuant
to Section 4.4, the Company shall continue to pay to the Employee the salary set
forth in Section 3.1, and shall continue to make available to the Employee the
benefits set forth in Section 3.3, excluding vacation days and bonus sums under
said Section 3.3 accrued during this period, until the later of (a) the first
anniversary of the Commencement Date of this Agreement, or (b) three (3) months
after the date of termination, but in no event later than


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such date as the Employee shall have commenced full-time employment with a new
employer.

          5.4  SURVIVAL. The provisions of Sections 6 and 7 shall survive the
termination of this Agreement.

     6.   NON-COMPETE.

          6.1  NON-COMPETE RESTRICTIONS During the Employment Period and for a
period of one (1) year after the termination or expiration thereof (such one (1)
year period being inapplicable in the event of a termination pursuant to Section
4.4 at the election of the Company or by the Employee pursuant to Section 4.5(a)
or (b)), the Employee will not directly or indirectly as an individual
proprietor, partner, officer, consultant, employee, director, joint venturer, or
in any other similar capacity engage in the business of developing, producing,
marketing or selling (or assist any other person engaging in the business of
developing, producing, marketing or selling) any Competitive Products. For this
purpose, "Competitive Products" shall mean any drugs or devices used to
diagnose, prevent or treat arterial or venous thrombosis, including, without
limitation:

-    Factor 7 inhibitors

-    Factor Xa inhibitors

-    Indirect thrombin inhibitors (e.g. heparin, any low molecular weight or
     otherwise fractionated heparin etc.)

-    Direct thrombin inhibitors (e.g. hirudins [Revasc(R), Refludan(R)],
     argatroban [Novastan], malagatran, naragaptran etc.)

-    Orally active platelet inhibitors including aspirin, ADP antagonists (e.g.
     Plavix(R), Ticlid(R)) oral GP2b-3a inhibitors etc.)

-    Intravenous platelet inhibitors (e.g. GP2b-3a inhibitors, Reopro(R),
     Aggrastat(R), Integrelin(R) etc.)

-    Fibrinolytics (e.g. streptokinase, urokinase, tPA, tPA analogues such as
     TNK, staphylokinase etc.).


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          6.2  LIMITATIONS. Notwithstanding the provisions of Section 6.1, it is
recognized that the Employee's primary experience is in the pharmaceutical
industry, and that his ability to earn a livelihood is likely to be dependent on
future employment in such industry. Accordingly, the Company agrees that the
employment of the Employee by a pharmaceutical company in a position in which he
assumes responsibility for multiple products, most of which are not Competitive
Products, shall not be considered a violation of Section 6.1, so long as (i) the
Employee's responsibilities in such position are not directed principally to
Competitive Products, (ii) the portfolio of the pharmaceutical company which
hires the Employee must contain the Competitive Products prior to the hiring of
the Employee, and (iii) the pharmaceutical company which hires the Employee has
been in existence for at least five (5) years prior to hiring the Employee.

          6.3  EXTENSION. If the Employee violates the provisions of Section
6.1, the Employee shall continue to be bound by the restrictions set forth in
this Section 6 until a period of one year has expired without any violation of
such provision.

          6.4  CUTBACK CLAUSE. 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.

          6.5  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 Employee to be reasonable for such purpose. The Employee
agrees that any breach of Section 6.1 is likely to cause the Company substantial
and irrevocable damage and therefore, in the event of any such breach, the
Employee agrees that the


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Company, in addition to such other remedies which may be available, shall be
entitled to specific performance and other injunctive relief.

     7.   NON-SOLICITATION.

          7.1  NON-SOLICITATION RESTRICTIONS. While the Employee is employed by
the Company and for a period of one (1) year after the termination or cessation
of such employment for any reason, the Employee will not directly or indirectly
recruit, solicit or hire any employee of the Company, or induce or attempt to
induce any employee of the Company to terminate his/her employment with, or
otherwise cease his/her relationship with, the Company (other than David Stack,
Fred Ryan, or Melinda Popolla). If the Employee violates the provisions of this
Section 7.1, the Employee shall continue to be bound by the restrictions set
forth in this Section 7.1 until a period of one (1) year has expired without any
violation of such provisions.

          7.2  CUTBACK CLAUSE. If any restriction set forth in Section 7.1 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 or range of activities as to which it may be enforceable.

          7.3  EQUITABLE REMEDIES. The restrictions contained in Section 7.1 are
necessary for the protection of the business and goodwill of the Company and are
considered by the Employee to be reasonable for such purpose. The Employee
agrees that any breach of Section 7 is likely to cause the Company substantial
and irrevocable damage and therefore, in the event of any such breach, the
Employee agrees that the Company, in addition to such other remedies which may
be available, shall be entitled to specific performance and other injunctive
relief.


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     8.   INVENTIONS AND PROPRIETARY INFORMATION.

          8.1  INVENTIONS.

          (a) The Employee 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, for the benefit of the Company (all of which are collectively referred
to in this Agreement as "Developments") during the Employment Period which
Developments are directly related to the services performed by the Employee
hereunder.

          (b) The Employee agrees to assign and does hereby assign to the
Company (or any person or entity designated by the Company) all its right, title
and interest in and to all Developments and all related patents, patent
applications, copyrights and copyright applications.

          (c) Notwithstanding the foregoing, this Agreement shall not require
assignment or disclosure of any Developments that: (i) the Employee develops
outside of this Agreement without using the Company's equipment, supplies,
facilities, or Proprietary Information (as defined below); (ii) do not directly
result from the specific services that the Employee performs for the Company
during the Employment Period; or (iii) do not relate to the business or research
of the Company, as disclosed to the Employee as of the date of this Agreement,
which disclosure may be modified or supplemented by written notice to the
Employee from time to time.

          (d) The Employee agrees to cooperate fully with the Company, both
during and after the term of this Agreement, with respect to the procurement,
maintenance and enforcement of copyrights, patents and other intellectual
property rights


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(both in the United States and foreign countries) relating to Developments,
except as provided above. The Employee 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 Developments.

          8.2  PROPRIETARY INFORMATION.

          (a) The Employee 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, "the
Company's Proprietary Information") which is disclosed to the Employee is and
shall be the exclusive property of the Company. By way of illustration, but not
limitation, the Company's Proprietary Information may include inventions,
products, 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 Employee
will not disclose any of the Company's Proprietary Information to any person or
entity other than employees of the Company on a need-to-know basis under this
Agreement or use the same for any purposes (other than in the performance of its
duties) without written approval by an officer of the Company, either during or
after the term of this Agreement unless and until such of the Company's
Proprietary Information has become public knowledge without fault by the
Employee.

          (b) The Employee agrees that all files, letters, memoranda, reports,
records, data, sketches, drawings, laboratory notebooks, program listings or
other written, photographic or other tangible material containing the Company's
Proprietary Information relating to the services performed hereunder, whether
created by the


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Employee or others during the Employment Period, shall be and are the exclusive
property of the Company. All such materials or copies thereof and all tangible
property of the Company in the custody or possession of the Employee shall be
delivered to the Company upon the earlier of (i) a request by the Company; or
(ii) termination of this Agreement. After such delivery, the Employee shall not
retain any such materials or copies thereof or any such tangible property,
except one copy for archival purposes.

          (c) The parties agree that their obligation not to disclose or to use
information and materials of the types set forth in paragraphs (a) and (b)
above, and their 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 or suppliers of the Company, or other third
parties who may have disclosed or entrusted the same to the Company.

          (d) The parties' obligations under this Section 8.2 shall not apply to
any information that (i) is or becomes known to the general public under
circumstances involving no breach by the parties of the terms of this Section
8.2; (ii) is generally disclosed to third parties by the Company without
restriction on such third parties; (iii) was known to Company or the Employee as
of the date of this Agreement as evidenced by documents in possession of Company
or the Employee; or (iv) is approved for release by written authorization of the
Company or the Employee.

          (g) The Employee represents that his employment by the Company and his
performance under this Agreement does not, and shall not breach any agreement
that obligates him to keep in confidence any of his trade secrets or
confidential or proprietary information or of any other party or to refrain from
competing, directly or indirectly, with


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the business of any other party. The Employee shall not disclose to the Company
any trade secrets or confidential or proprietary information of any other party.

          (h) The Employee acknowledges that as of the date hereof the Company
has agreements with other persons or with the United States Government, or
agencies thereof, that 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 Employee agrees to be bound
by all such obligations and restrictions that are known to it and to take all
action necessary to discharge the obligations of the Company under such
agreements.

          8.3  REMEDIES. The parties acknowledge that any breach of the
provisions of this Section 8 shall result in serious and irreparable injury to
the non-breaching party for which the parties cannot be adequately compensated
by monetary damages alone. The parties agree, therefore, that, in addition to
any other remedy they may have, the Company shall be entitled to enforce the
specific performance of this Agreement against the Employee and to seek both
temporary and permanent injunctive relief (to the extent permitted by law)
without the necessity of proving actual damages in any federal or state court
within Massachusetts and the Employee shall be entitled to enforce the specific
performance of the Agreement against the Company and to seek both temporary and
permanent injunctive relief (to the extent permitted by law) without the
necessity of proving actual damages in any federal or state court within
Massachusetts.

     9.   OTHER AGREEMENTS. Employee hereby represents that he is not bound by
the terms of any agreement with any previous employer or other party to refrain
from using or disclosing any trade secret or confidential or proprietary
information in the course of his employment with the Company or to refrain from
competing, directly or


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indirectly, with the business of such previous employer or any other party.
Employee further represents that his performance of all the terms of this
Agreement and as an employee of the Company does not and will not breach any
agreement to keep in confidence proprietary information, knowledge or data
acquired by him in confidence or in trust prior to his employment with the
Company.

     10.  NOTICES. All notices required or permitted under this Agreement shall
be in writing and shall be deemed effective upon personal delivery or upon
deposit in the United States Post Office, by registered or certified mail,
postage prepaid, addressed to the other party at the address shown above, or at
such other address or addresses as either party shall designate to the other in
accordance with this Section 9.

     11.  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.

     12.  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.
Reference is also made to the Non-Statutory Stock Option Agreement between the
Employee and the Company dated of even date herewith.

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

     14.  GOVERNING LAW. This Agreement shall be construed, interpreted and
enforced in accordance with the laws of the Commonwealth of Massachusetts.

     15.  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


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entity 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
Employee are personal and shall not be assigned by him.

     16.  MISCELLANEOUS.

          16.1 NO WAIVER. 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.

          16.2 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.

          16.3 ENFORCEABILITY. 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.

          16.4 COUNTERPARTS. This Agreement may be executed in one or more
counterparts, each of which shall be deemed to be an original, but all of which
shall be one and the same document.

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

                                          THE MEDICINES COMPANY


                                          By: /s/ Clive A. Meanwell
                                             ----------------------------------
                                          Title: President
                                                -------------------------------




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<PAGE>   16

                                             /s/ Thomas Quinn
                                            -----------------------------------
                                            Thomas Quinn



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<PAGE>   17
                      MODIFICATION TO EMPLOYMENT AGREEMENT


     This Modification to the Employment Agreement dated April 1, 2000 by and
between The Medicines Company, a Delaware Corporation with its principal place
of business at One Cambridge Center, Cambridge, Massachusetts 02142 and Thomas
Quinn, residing at 325 Independence Drive, Orchard Park, New York 14127 is as
follows:

     New paragraphs 4.5(iii) and 4.5(iv) are added as set forth below:

     (iii) David Stack shall have ceased to serve as Senior Vice President of
     the Company; or

     (iv) a failure by Employer to provide infrastructure support either under
     the Stack Pharmaceutical Services Agreement dated April 1, 2000 or through
     other means acceptable to Employee.

     IN WITNESS WHEREOF, the parties hereto have executed this Modification to
Employment Agreement this 13th day of April, 2000.

                                          THE MEDICINES COMPANY



                                          By: /s/ Clive A. Meanwell
                                             ---------------------------------
                                          Title: Chief Executive Officer
                                                ------------------------------


                                              /s/ Thomas Quinn
                                              --------------------------------
                                              Thomas Quinn
<PAGE>   18
                                    AGREEMENT

     This Amendment to Employment Agreement (the "Amendment"), made as of the
1st day of June, 2000, is entered into by The Medicines Company, a Delaware
corporation with its principal place of business at One Cambridge Center,
Cambridge, Massachusetts 02142 (the "Company"), and Thomas Quinn, residing at
325 Independence Drive, Orchard Park, New York 14127 (the "Employee").

     WHEREAS, the Company and the Employee are parties to an Employment
Agreement dated as of April 1, 2000, as amended by a modification dated April
13, 2000 (the "Employment Agreement"); and

     WHEREAS, the Company and the Employee desire to make certain amendments to
the terms of the Employment Agreement;

     NOW THEREFORE, in consideration of these premises, the mutual covenants and
promises contained herein, and other good and valuable consideration, the
receipt and sufficiency of which are acknowledged, the parties hereto agree as
follows:

I.   Employment Agreement

     1.   DEFINED TERMS. The capitalized terms used herein but not otherwise
defined shall have the meanings assigned to them in the Employment Agreement.

     2.   TITLE; CAPACITY. The Employment Agreement is hereby amended by
deleting Section 2 of the Employment Agreement in its entirety and inserting in
lieu thereof the following:

                    "2.  TITLE; CAPACITY. The Employee shall serve as Vice
                         President or in such other position as the Company or
                         its Board of Directors (the "Board") may determine from
                         time to time The Employee shall be based at the offices
                         of Stack Pharmaceuticals, Inc., 5 Sylvan Way,
                         Parsippany, New Jersey, unless otherwise agreed by the
                         parties. The Employee shall be subject to the
                         supervision of, and shall have such authority as is
                         delegated to him by David Stack, Senior Vice President
                         of the Company (or such other officer of the Company as
                         may be designated by the Board).

                              The Employee hereby accepts such employment and
                         agrees to undertake the duties and responsibilities
                         inherent in such position and/or such other duties and
                         responsibilities as David Stack (or the Board or its
                         designee) shall from time to time reasonably assign to
                         him. The Employee agrees to devote his or her full
                         business time, attention and energies to the business
                         and interests of the Company during the Employment
                         Period. The Employee 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

<PAGE>   19

                         except as provided in these agreements. The Employee
                         acknowledges receipt of copies of all such rules and
                         policies committed to writing as of the date of this
                         Agreement."

     3.   SALARY. Section 3.1 of the Employment Agreement is hereby amended by
deleting the amount "$120,000" and inserting in lieu thereof "$190,000."

     4.   BONUS. Section 3.2 of the Employment Agreement is hereby amended by
deleting "30%" and inserting in lieu thereof "40%."

II.  Miscellaneous Provisions

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

     2.   This Amendment shall be construed, interpreted, and enforced in
accordance with the laws of the Commonwealth of Massachusetts, without giving
effect to conflict of laws provisions.

     3.   This Amendment, together with the Employment Agreement, constitute the
entire agreement between the parties and supersedes all prior agreements and
understandings, whether written or oral, relating to the subject matter hereof.

     4.   In all respects other than as specifically provided in this Amendment,
the Employment Agreement is hereby ratified and affirmed.

                     [Remainder of page intentionally blank]

                                     - 2 -
<PAGE>   20

     In witness whereof, the parties hereto have executed this Amendment as of
the day and year set forth above.


                                        THE MEDICINES COMPANY


                                   By:  /s/ Clive A. Meanwell
                                       -----------------------------------------

                                   Title:  President and Chief Executive Officer
                                          --------------------------------------

                                   EMPLOYEE

                                    /s/ Thomas Quinn
                                   ---------------------------------------------
                                   Thomas Quinn


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