Document:

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                                                                    EXHIBIT 10.3

                                     FORM OF
                      PRAECIS PHARMACEUTICALS INCORPORATED
                          EMPLOYEE STOCK PURCHASE PLAN

         PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware corporation (the
"Company"), establishes this Employee Stock Purchase Plan (the "Plan") so that
Eligible Employees (as defined herein), if any, may be granted options to
purchase Common Stock, par value $.01 per share, of the Company ("Common
Stock").

1.      PURPOSE.

         The Plan provides Eligible Employees an opportunity to acquire shares
of Common Stock under circumstances which enable them to obtain the income tax
benefits described in Section 423 of the Internal Revenue Code of 1986, as
amended (the "Code"). The Plan is intended to provide employees an incentive to
continue to promote the Company's best interests and to enhance its long-term
performance.

2.      DEFINITIONS.

         Wherever used, the following words and phrases will have the meanings
stated below unless a different meaning is plainly required by the context:

"Affiliated Company" means any subsidiary corporation of the Company, as defined
in Section 424(f) of the Code.

"Applicable Grant Date" means for any Option the date on which such Option was
granted, which shall be a Semiannual Grant Date.

"Board" means the Board of Directors of the Company.

"Committee" means a committee appointed by the Board to which the Board may (but
shall not be required to) delegate its powers to administer the Plan.

"Compensation" means the total cash remuneration a Participant receives during
an Exercise Period as salary or wages, including overtime pay and bonuses and
excluding all other forms of remuneration.

"Disability" means permanent and total disability as defined in Section 22(e)(3)
of the Code.

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"Effective Date" means July 3, 2000.

"Eligible Employee" means each person who, on an applicable Semiannual Grant
Date, is employed by the Company or an Affiliated Company and has been an
employee for six (6) or more months at that date. An employee will not be
eligible to participate during an Exercise Period if his or her customary
employment as of the first day of the period is either (i) less than 20 hours
per week or (ii) 5 months or less on a calendar year basis. No employee will be
eligible if he or she is on an applicable Semiannual Grant Date an owner of 5%
or more of the outstanding capital stock of the Company or an Affiliated
Company, as determined under Section 424(d) of the Code.

"Exercise Date" means any date on which an Eligible Employee purchases Common
Stock pursuant to an Option under the Plan, which shall, with respect to each
Option, be the last day of the Exercise Period in which such Option is granted.

"Exercise Period" means the approximate six-month period commencing on each of
July 3, 2000, January 1, 2001, July 2, 2001 and December 31, 2001 and ending at
5 p.m. (Boston time) on each of December 31, 2000, July 1, 2001, December 30,
2001 and June 30, 2002, respectively. If the Plan is terminated, then the
Exercise Period in which it is terminated shall end on the date immediately
preceding the effective date of such termination.

"Fair Market Value Per Share of Common Stock" on the day of determination shall
mean the last sale price per share of Common Stock on such day reported on the
Nasdaq National Market as published in The Wall Street Journal or, if no such
sale is so reported, the average of the reported closing bid and asked prices on
such day in the over-the-counter market, as furnished by the National
Association of Securities Dealers Automated Quotation System, or, if such price
at the time is not available from such system, as furnished by any similar
system then engaged in the business of reporting such prices and selected by the
Board or, if there is no such system, as furnished by any member of the National
Association of Securities Dealers, selected by the Board. If the Common Stock is
neither reported on the Nasdaq National Market nor traded on the
over-the-counter market, fair market value shall be such value as the Board, in
good faith, determines. Notwithstanding any provision of the Plan to the
contrary, no determination made with respect to the Fair Market Value Per Share
of Common Stock subject to an Option shall be inconsistent with Section 423 of
the Code.

"Initial Notice Period" means the period beginning on the Effective Date and
ending on the 15th day thereafter.

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"Notice Period" means that period beginning 30 days prior to the applicable
Semiannual Grant Date (other than the first Semiannual Grant Date) and ending
on the 15th day prior to said date.

"Option" means an option granted hereunder which will entitle an Eligible
Employee to purchase shares of Common Stock.

"Option Price" means the lower of: (1) 85% of the Fair Market Value Per Share of
Common Stock as of the Applicable Grant Date on which the Option being exercised
was granted or (2) 85% of the Fair Market Value Per Share of Common Stock as of
the Exercise Date on which such Option is exercised.

"Participant" means an Eligible Employee who has elected to participate in the
Plan during the period between such election and the termination of such
Eligible Employee's participation in the Plan.

"Retirement" means a termination of a Participant's employment with the Company
on or after the first day of the month of a Participant's 65th birthday.

"Semiannual Grant Date" means each of July 3, 2000, January 1, 2001, July 2,
2001 and December 31, 2001.

"Withholding Account" means a bookkeeping record of all amounts withheld during
an Exercise Period for a specific Eligible Employee, which are available for the
exercise of an Option granted hereunder. Specific segregation of funds is not
required.

3.       ADMINISTRATION.

         The Plan shall be administered by the Board, which, to the extent it
shall determine, may delegate its powers with respect to the administration of
the Plan (except its powers to terminate or amend the Plan) to the Committee. If
the Board chooses to appoint a Committee, references hereinafter to the Board
shall be deemed to refer to the Committee. Subject to the express provisions of
the Plan, the Board may interpret the Plan, prescribe, amend and rescind rules
and regulations relating to it, determine the terms and provisions of the
Options granted hereunder and make all other determinations necessary or
advisable for the administration of the Plan; provided, however, that all such
interpretations, rules, determinations, terms and conditions shall be made and
prescribed in the context of preserving the tax treatment of the Options under
this Plan granted to Eligible Employees subject to United States federal income
taxation and preserving the tax treatment of the Plan itself under Section 423
of the Code. The determinations of the Board on all matters regarding the Plan
shall be conclusive.

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4.       MAXIMUM SHARES TO BE GRANTED UNDER THE PLAN.

         The aggregate number of shares of Common Stock available for issuance
upon the exercise of Options granted pursuant to Section 5 shall be Forty
Thousand (40,000) as of July 3, 2000, with an additional Forty Thousand
(40,000) shares available as of each of January 1, 2001, July 2, 2001 and
December 31, 2001, subject to adjustment as set forth below or pursuant to
Section 9. Shares of Common Stock delivered pursuant to the Plan will be
authorized but unissued shares or treasury shares. In the event that any Option
granted pursuant to Section 5 expires or is terminated, surrendered or cancelled
without being exercised, in whole or in part, for any reason, the number of
shares of Common Stock theretofore subject to such Option shall again be
available as shares underlying Options which may be granted for grant at the
next Semiannual Grant Date pursuant to Section 5 and shall increase the
aggregate number of shares of Common Stock underlying Options available for
grant during the succeeding Exercise Period.

5.       ELIGIBILITY FOR PARTICIPATION AND GRANTING OF OPTIONS.

         (a) Each employee of the Company who enrolls in the Plan and who is an
Eligible Employee on an applicable Semiannual Grant Date is granted without any
further action by the Board an Option hereunder which will entitle him or her to
purchase, on the immediately following Exercise Date at the Option Price per
share for Options granted on such date, shares of Common Stock equal in value up
to ten percent (10%) of the Eligible Employee's Compensation during the Exercise
Period divided by such applicable Option Price per share of Common Stock.

         (b) If the number of shares of Common Stock for which Options are
granted pursuant to Section 5(a) exceeds the applicable number provided for in
Section 4, then the Options granted under Section 5(a) to all Eligible Employees
shall, in a nondiscriminatory manner, be reduced on a pro rata basis in a manner
which the Board determines to be consistent with Section 423 of the Code.

         (c) No Eligible Employee shall be granted an Option under the Plan
which permits his or her rights to purchase stock under all employee stock
purchase plans (as defined in Section 423 of the Code) of the Company and any
Affiliated Company to accrue at a rate which exceeds $25,000 of fair market
value of such stock (determined at the time of the grant of such Option) for
each calendar year in which such Option is outstanding at any time. Any Option
granted under the Plan shall be deemed to be modified to the extent necessary to
satisfy this Section 5(c).

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6.       TERMS OF OPTIONS.

         (a) Each Option shall automatically be exercised on the last day of the
Exercise Period for such Option, using the funds which have accrued in a
Participant's Withholding Account as of such day, unless the Participant
withdraws from the Plan or is deemed to withdraw during the Exercise Period. An
Option granted hereunder may be exercised only through the use of the funds
which have accrued in a Participant's Withholding Account. Any Option, to the
extent unexercised on the Exercise Date, shall expire on the Exercise Date.

         (b) As soon as reasonably possible following exercise in accordance
with Section 6(a) and upon the Participant's written request, a certificate
representing the whole number of shares of Common Stock purchased, registered in
the name of the Participant, shall be delivered to the Participant or to such
other person designated by the Participant, including, without limitation, the
Participant's broker.

         (c) A Participant shall be deemed to have withdrawn from participation
in the Plan upon the occurrence of any of the following events:

                  (i) VOLUNTARY DISCONTINUANCE WHILE EMPLOYED. A Participant
may discontinue his or her election and withdraw from the Plan by giving written
notice to the Company no later than the last day of the Notice Period within
that Exercise Period, specifying that the Participant is so withdrawing from the
Plan; provided, however, that a Participant who shall have discontinued his or
her election to participate and withdrawn from the Plan may not participate in
the Plan during the next following Exercise Period.

                  (ii) TERMINATION OF EMPLOYMENT. Unless employment has
terminated due to Retirement, Disability or death, a Participant will be deemed
to have discontinued participation on the first day of the Exercise Period in
which such Participant's termination of employment occurs and amounts withheld
from compensation during the Exercise Period will be refunded.

                  (iii) RETIREMENT. In the event a Participant's employment
terminates because of Retirement during the first three months of an Exercise
Period, the Participant will be deemed to have discontinued participation on the
first day of the Exercise Period in which Retirement occurs and amounts withheld
from Compensation during the Exercise Period will be refunded. If Retirement
occurs during the last three months of the Exercise Period, the Participant will
continue to participate through the balance of the Exercise Period in which
Retirement occurs (without further withholding) unless he or she elects a
voluntary discontinuance within the Notice Period for that Exercise Period.

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                  (iv) DEATH OR DISABILITY. In the event the employment of the
Participant by the Company or an Affiliated Company terminates as a result of
the Participant's Disability or death, the Participant will be deemed to
participate (without further withholding) through the balance of the Exercise
Period in which death or Disability occurs, unless he or she (or the executor,
administrator or representative, as the case may be) elects a voluntary
discontinuance within the Notice Period for that Exercise Period.

                  (v) LEVY OR ATTACHMENT. The filing with or levying upon the
Company or the custodian of any judgment, attachment, garnishee or other court
order affecting the Participant's account under the Plan will automatically
terminate such Participant's participation in the Plan and amounts withheld from
compensation during the Exercise Period will be refunded.

                  (vi) PLAN TERMINATION/EXPIRATION. Subject to Section 12(b),
termination of the Plan will terminate the participation of all Participants in
the Plan.

         (d) A Participant's employment shall not be deemed terminated by reason
of a transfer to another employer which is related to the Company within the
meaning of Sections 424(e) or (f) of the Code. A Participant who has elected
participation under the Plan who is absent from work with the Company or with an
Affiliated Company because of temporary disability (any disability other than a
permanent and total Disability) or who is on leave of absence for a period of
less than 90 days shall not, during the period of any such absence, be deemed,
by virtue of such absence alone, to have terminated employment. In the case of a
leave of absence which is longer than 90 days, a Participant will not be deemed
to have terminated employment until the later of the 91st day of such leave or,
if later, such date as the Participant's reemployment rights are not protected
by contract or law.

         (e) Upon the discontinuance of an election and withdrawal from the Plan
by a Participant, all withheld amounts in such Participant Withholding Account
shall be transferred to such Participant within thirty (30) days of such
discontinuance and withdrawal, except to the extent such withheld amounts are
applied to the exercise of an Option as provided above. In no event shall any
amounts be withheld from a Participant's Compensation for allocation to such
Participant's Withholding Account after the date such Participant's employment
shall cease.

         (f) In no event may any discontinuance of a Participant's election and
withdrawal from the Plan be in respect to a portion rather than all of such
Participant's Withholding Account on such date.

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7.       PAYMENT FOR COMMON STOCK THROUGH WITHHOLDING.

         (a)  EMPLOYEE CONTRIBUTIONS

         Each Eligible Employee may elect to participate in the Plan by filing
an enrollment application and payroll withholding form with his or her
employer's payroll department during the Initial Notice Period or during a
Notice Period, which election shall be effective in the case of an election
filed during the Initial Notice Period, for the Exercise Period commencing on
the Effective Date and all subsequent Exercise Periods, or, in the case of an
election filed during a Notice Period, for the next Exercise Period and for all
subsequent Exercise Periods, until, in any case, such Participant's
participation in the Plan terminates. Each Eligible Employee who elects to
participate shall specify the amount of his or her contributions to be made by
payroll deduction by specifying a whole percentage from 1% to 10% of such
Participant's Compensation payable for each payroll period.

         No interest shall accrue or be payable to any Participant in the Plan
with respect to any sums withheld at the Participant's election, whether such
sums be applied to purchase Common Stock, or are returned to the Participant.

         Payroll deductions may be increased by a Participant only during a
subsequent Notice Period, but may be decreased, upon the Participant's written
election, effective as of the first payroll period for which it is
administratively practical to put the decrease into effect.

         (b)  APPLICATION OF PAYROLL CONTRIBUTIONS

         The Company shall maintain a separate account into which it shall
deposit all amounts withheld for payment of shares of Common Stock and shall
maintain sufficient records reflecting each Participant's Withholding Account.

         On the last day of each Exercise Period all amounts in a Participant's
With holding Account shall be paid over to the Company in payment of the Option
Price for the number of whole shares of Common Stock which can be purchased on
such date with such withheld total amount, unless otherwise directed in
accordance with Section 6 above. In lieu of fractional shares, unapplied cash
shall be carried forward to the next Exercise Period unless the Participant
requests a cash payment.

8.       TRANSFERABILITY OF OPTIONS AND COMMON STOCK.

         (a) No Option may be transferred, assigned, pledged or hypothecated
(whether by operation of law or otherwise), except as provided by will or the
applicable laws of descent or distribution, and no Option shall be subject to
execu-

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tion, attachment or similar process. Any attempted assignment, transfer, pledge,
hypothecation or other disposition of an Option, or levy of attachment or
similar process upon the Option not specifically permitted herein shall be null
and void and without effect. An Option may be exercised only by the Eligible
Employee during his or her lifetime, or by his or her legal representative if
permitted by Section 423 of the Code, or pursuant to Section 6 by his or her
estate or the person who acquires the right to exercise such Option upon his or
her death by bequest or inheritance.

         (b) Participants in the Plan who wish to avail themselves of the
favorable tax benefits of Section 423 of the Code may not transfer or otherwise
dispose of shares of Common Stock acquired by them or on their behalf under the
Plan (other than in the case of a Participant's death) until after the later of
one year from the date of acquisition of said shares and two years after the
Applicable Grant Date of the Option pursuant to which said shares of Common
Stock were acquired.

         (c) Each Eligible Employee who receives shares of Common Stock pursuant
to the Plan agrees, by electing to participate, to notify the Company, in
writing, immediately after such Participant makes a Disqualifying Disposition of
any shares acquired pursuant to the exercise of an Option under the Plan. A
Disqualifying Disposition is any disposition (including any sale) of such shares
before the later of two years after the Applicable Grant Date for said Option or
one year after the receipt of shares pursuant to the exercise of said Option. If
the Participant has died before such stock is sold, these holding period
requirements do not apply and no Disqualifying Disposition can occur
thereafter.

9.       ADJUSTMENT PROVISIONS.

         The aggregate number of shares of Common Stock with respect to which
Options may be granted, the aggregate number of shares of Common Stock subject
to each outstanding Option and the Option Price per share of each Option shall
all be appropriately adjusted if any of the following occur after the Plan's
adoption by the Board: any increase or decrease in the number of shares of
issued Common Stock resulting from a subdivision or consolidation of shares,
whether through reorganization, recapitalization, stock split, stock
distribution or combination of shares, or the payment of a share dividend or
other increase or decrease in the number of such shares outstanding effected
without receipt of consideration by the Company. Adjustments shall be made
according to the sole discretion of the Board, and its decision shall be binding
and conclusive.

10.      DISSOLUTION, MERGER AND CONSOLIDATION.

         Upon the dissolution or liquidation of the Company, or upon a merger or
consolidation of the Company in which the Company is not the surviving corpora-

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tion, the holder of each Option then outstanding under the Plan will thereafter
be entitled to receive at the next Exercise Date upon the exercise of such
Option for each share of Common Stock as to which such Option shall be
exercised, as nearly as reasonably may be determined, the cash, securities
and/or property which a holder of one share of the Common Stock was entitled to
receive upon and at the time of such transaction. The Board shall take such
steps in connection with such transactions as the Board shall deem necessary to
assure that the provisions of this Section 10 shall thereafter be applicable, as
nearly as reasonably may be determined, in relation to the said cash, securities
and/or property as to which such holder of such Option might thereafter be
entitled to receive.

11.      STOCKHOLDER APPROVAL.

         The Plan is subject to approval by the holders of a majority of the
outstanding shares of Common Stock (and the holders of any other class of stock
to the extent required by law, agreement or Section 423 of the Code) within 12
months before or after the date of adoption of the Plan by the Board. The Plan
shall be null and void and of no effect if the foregoing condition is not
fulfilled.

12.      MISCELLANEOUS.

         (a) LEGAL AND OTHER REQUIREMENTS. The obligations of the Company to
sell and deliver Common Stock under the Plan shall be subject to all applicable
laws, regulations, rules and approvals, including, but not by way of limitation,
the effectiveness of a registration statement under the Securities Act of 1933,
as amended, if deemed necessary or appropriate by the Company. Certificates for
shares of Common Stock issued hereunder may be legended to such effect as the
Board shall deem appropriate.

         (b) TERMINATION AND AMENDMENT OF PLAN. Except as provided in the
following sentence, the Plan may be terminated or amended by the stockholders of
the Company, by the Board, or by the Committee, including amendment of the Plan
from time to time to designate corporations whose employees may be offered
options under the Plan from among a group consisting of the Company and any
corporation which is or becomes its Affiliate. Amendments effecting: (1) any
increase in the aggregate number of shares of Common Stock which may be issued
under the Plan (other than an increase merely reflecting a change in
capitalization such as a stock dividend or stock split) or (2) changing the
designation of corporations whose employees may be offered options under the
Plan, except designations described in the preceding sentence, must be approved
by the stockholders of the Company within twelve (12) months after such
amendment is adopted by the Board or by the Committee or such amendment is void
ab initio. No amendment to the Plan shall affect any Options theretofore granted
or any Common Stock theretofore acquired by a

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Participant, unless such amendment shall expressly so provide and unless any
Participant to whom an Option has been granted who would be adversely affected
by such amendment consents in writing thereto. Unless otherwise determined by
the Board, no Options will be granted under the Plan after December 31, 2001.
The Plan shall expire on June 30, 2002, unless extended or earlier terminated in
accordance with the terms hereof.

         (c) WITHHOLDING TAXES. Upon a Disqualifying Disposition, within the
meaning of Section 8(d), of any shares of Common Stock received pursuant to the
exercise of any Option under the Plan, the Company shall have the right to
require the Participant to remit to the Company an amount sufficient to satisfy
all federal, state and local requirements as to income tax withholding and
employee contributions to employment taxes or, alternatively, in the Board's
sole discretion, the Company may withhold all such amounts from other
compensation due to the Participant by the Company.

         (d) RIGHT TO TERMINATE EMPLOYMENT. Nothing in the Plan or any agreement
entered into pursuant to the Plan shall confer upon any Eligible Employee or
other optionee the right to continue in the employment of the Company or any
Affiliated Company or affect any right which the Company or any Affiliated
Company may have to terminate the employment of such Eligible Employee or other
optionee.

         (e) RIGHTS AS A STOCKHOLDER. A Participant shall not have any right as
a stockholder of the Company with respect to shares of Common Stock issuable
pursuant to the exercise of an Option hereunder, unless and until a certificate
or certificates for such shares of Common Stock are issued to him or her or the
Company reflects the Participant's ownership in its stock ledger or other
appropriate record of Common Stock ownership.

         (f) LEAVES OF ABSENCE. The Board shall be entitled to make such rules,
regulations and determinations as it deems appropriate under the Plan in respect
of any leave of absence taken by any Eligible Employee, provided such rules are
consistent with Section 423 of the Code.

         (g) NOTICES. Every direction, revocation or notice authorized or
required by the Plan shall be deemed delivered to the Company (1) on the date it
is personally delivered to the Treasurer of the Company (or such other person as
may be designated by the Company from time to time with notice given to each
Participant) at its principal executive offices or (2) three business days after
it is sent by registered or certified mail, postage prepaid, addressed to the
Treasurer of the Company (or such other person as may be designated by the
Company from time to time with notice given to each Participant) at such offices
or (3) on the date on which delivery was guaranteed by a third party business
(such as Federal Express and including the

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postage service); and shall be deemed delivered to a Participant (A) on the date
it is personally delivered to him or her or (B) three business days after it is
sent by registered or certified mail, postage prepaid, addressed to him or her
at the last address shown for him or her on the records of the Company or of any
Affiliate or (C) on the date on which delivery was guaranteed by a third party
business (such as Federal Express and including the postal service), provided
that the documents were sent to him or her at the last address shown for him or
her on the records of the Company or of any Affiliate.

         (h) All Eligible Employees shall have the same rights and privileges
under the Plan, except that the amount of Common Stock which may be purchased
under Options granted under the Plan shall bear a uniform relationship to the
Compensation of Eligible Employees. All rules and determinations of the Board in
the administration of the Plan shall be uniformly and consistently applied to
all persons in similar circumstances.

         (i) APPLICABLE LAW. All questions pertaining to the validity,
construction and administration of the Plan and Options granted hereunder shall
be determined in conformity with the laws of the Commonwealth of Massachusetts,
to the extent not inconsistent with Section 423 of the Code and the regulations
thereunder.

                                       11<PAGE>

                                                                    Exhibit 10.5

                      STOCK AND WARRANT PURCHASE AGREEMENT

                                     between

                                SYLAMERICA, INC.

                                       and

                          PRAECIS PHARMACEUTICALS, INC.

                            Dated as of May 13, 1997

<PAGE>

                                TABLE OF CONTENTS
                                -----------------

<TABLE>
<CAPTION>

<S>                                                                                                 <C>
ARTICLE I  PURCHASE AND SALE OF SHARES AND WARRANT....................................................1

         1.1      Purchase and Sale; Purchase Price...................................................1
         1.2      Closing and Closing Date............................................................1
         1.3      Deliveries at the Closing...........................................................1

ARTICLE II  REPRESENTATIONS AND WARRANTIES OF
                            PURCHASER.................................................................3

         2.1      Due Organization; Authority; Valid
                    Agreement.........................................................................3
         2.2      No Violation........................................................................4
         2.3      Investment Intention................................................................5
         2.4      Disclosure of Information...........................................................5
         2.5      Investment Experience; Accredited
                    Investor..........................................................................5
         2.6      Restricted Securities...............................................................6
         2.7      Further Limitations on Disposition..................................................7
         2.8      Legends; Stop Transfer Instructions.................................................8

ARTICLE III  REPRESENTATIONS AND WARRANTIES OF
                           PRAECIS...................................................................11

         3.1      Due Organization; Authority; Valid
                    Agreement........................................................................12
         3.2      No Violation.......................................................................13
         3.3      Validity of Shares and Warrant Shares..............................................14
         3.4      Financial Statements...............................................................15
         3.5      Capitalization.....................................................................18
         3.6      Litigation.........................................................................19
         3.7      Intellectual Property..............................................................19
         3.8      Registration Rights................................................................20
         3.9      Corporate Documents................................................................20
         3.10     Title to Property and Assets.......................................................20
         3.11     Tax Returns and Payments...........................................................20
         3.12     Changes............................................................................22
         3.13     Contracts..........................................................................23
         3.14     Compliance with Laws...............................................................23
         3.15     Exempt Transaction.................................................................24
         3.16     Investment Company Act of 1940.....................................................24
</TABLE>

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<TABLE>
<CAPTION>

<S>                                                                                               <C>
ARTICLE IV  RESTRICTIONS ON TRANSFER OF THE
                           SECURITIES................................................................24

         4.1      Restrictions on Transfer of Warrant................................................24
         4.2      Restrictions on Transfer of Common
                    Stock............................................................................25

ARTICLE V  STANDSTILL................................................................................28

ARTICLE VI  REGISTRATION RIGHTS......................................................................32

         6.1      Piggyback Registration.............................................................32
         6.2      Registration Procedures............................................................36
         6.3      Indemnification....................................................................41
         6.4      Contribution.......................................................................47
         6.5      Rule 144...........................................................................49
         6.6      Definitions........................................................................50

ARTICLE VII  INDEMNIFICATION; CERTAIN COVENANTS......................................................52

         7.1      Indemnification....................................................................52
         7.2      Information to be Furnished........................................................55
         7.3      Closing Deliveries.................................................................55

ARTICLE VIII  ENFORCEMENT............................................................................56

         8.1      Injunctive Relief..................................................................56
         8.2      Consent to Jurisdiction and Venue..................................................56
         8.3      Service of Process.................................................................57

ARTICLE VIX  MISCELLANEOUS...........................................................................58

         9.1      Survival...........................................................................58
         9.2      Entire Agreement; No Assignment;
                    Severability.....................................................................59
         9.3      Notices............................................................................61
         9.4      Best Efforts.......................................................................62
         9.5      Fees and Expenses..................................................................63
         9.6      Certain Definitions................................................................63
         9.7      Counterparts.......................................................................64
         9.8      Governing Law......................................................................64

</TABLE>

                                       ii

<PAGE>

                      STOCK AND WARRANT PURCHASE AGREEMENT

                  STOCK AND WARRANT PURCHASE AGREEMENT dated as of May 13, 1997
(this "Agreement"), between Sylamerica, Inc., a Delaware corporation
("Purchaser") and a wholly owned subsidiary of Synthelabo, a societe anonyme
organized and existing under the laws of France ("Synthelabo"), and PRAECIS
PHARMACEUTICALS, INC., a Delaware corporation ("PRAECIS").

                  Concurrently with the execution and delivery of this
Agreement, PRAECIS and Synthelabo are entering into a definitive License
Agreement.
                  In consideration of the representations, covenants and
agreements herein contained, Purchaser and PRAECIS hereby agree as follows:

                                    ARTICLE I

                     PURCHASE AND SALE OF SHARES AND WARRANT

                  1.1 Purchase and Sale; Purchase Price. Upon the terms and
subject to the conditions set forth in this Agreement, Purchaser agrees to
purchase from PRAECIS, and PRAECIS agrees to sell to Purchaser, 215,703 shares
of Common Stock (the "Shares") and a five-year warrant to purchase 53,926 shares
of Common Stock at an exercise

<PAGE>

price of $96.6 per share (the "Warrant"), for an aggregate purchase price of
U.S. $10,000,000 (the "Purchase Price").

                  1.2 Closing and Closing Date. The closing of the purchase and
sale of the Shares and the Warrant hereunder (the "Closing") is occurring on the
date hereof (sometimes referred to as the "Closing Date") at the offices of
Skadden, Arps, Slate, Meagher & Flom LLP, One Beacon Street, Boston,
Massachusetts.

                  1.3 Deliveries at the Closing. At the Closing, PRAECIS is
delivering to Purchaser (i) a certificate or certificates, in definitive form,
representing the Shares, registered in Purchaser's name, (ii) a certificate
evidencing the Warrant in the form attached hereto as Exhibit A, duly executed
by PRAECIS and registered in Purchaser's name, and (iii) all other documents
required by the terms hereof to be delivered by PRAECIS at or prior to the
Closing against (x) payment by Purchaser to PRAECIS of the Purchase Price in
U.S. dollars in immediately available funds and (y) delivery by Purchaser to
PRAECIS of all other documents required by the terms hereof to be delivered by
Purchaser at or prior to the Closing.

                                        2

<PAGE>

                                   ARTICLE II
                   REPRESENTATIONS AND WARRANTIES OF PURCHASER

                  Purchaser hereby represents, warrants to, and
agrees with, PRAECIS as follows:

                  2.1 Due Organization; Authority; Valid Agreement. Purchaser 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 as a foreign
corporation in each jurisdiction in which such qualification is required, except
where the failure to so qualify would not have a material adverse effect on
Purchaser's ability to perform its obligations hereunder. Purchaser has the
requisite corporate power and authority to enter into and perform this Agreement
and to consummate the transactions contemplated hereby. The execution, delivery
and performance by Purchaser of this Agreement and the consummation by Purchaser
of the transactions contemplated hereby have been duly authorized by all
requisite corporate action on the part of Purchaser. This Agreement has been
duly executed and delivered by Purchaser and is a valid and binding agreement of
Purchaser, enforceable against Purchaser in accordance with its terms, subject
to (i) any applicable bankruptcy, insolvency, fraudulent conveyance,
reorganization, moratorium or

                                        3

<PAGE>

other similar laws now or hereafter in effect affecting creditors' rights
generally and (ii) general principles of equity (regardless of whether
enforcement is considered in a proceeding in equity or at law).

                  2.2 No Violation. The execution, delivery and performance by
Purchaser of this Agreement and the purchase of the Shares and the Warrant by
Purchaser pursuant to this Agreement does not and will not (i) violate or
conflict with or result in a breach of the terms, conditions or provisions of,
or constitute (or with notice or lapse of time, or both, constitute) a default
under (a) Purchaser's Certificate of Incorporation or By-laws as in effect on
the date hereof or (b) any mortgage, agreement, instrument, order, judgment,
injunction or decree to which Purchaser or its properties or assets is bound or
subject or (c) any statute, law, rule or regulation to which Purchaser or any of
its Affiliates (as defined herein) or their respective properties or assets is
bound or subject or (ii) require any consent, approval, authorization or permit
of, or filing with, any governmental or regulatory authority, to be obtained or
made by Purchaser or any of its Affiliates, except, in the case of clause
(i)(b), for any violation, conflict, breach or default which would not have a
material adverse effect on

                                        4

<PAGE>

the ability of Purchaser to perform its obligations hereunder.

                  2.3 Investment Intention. Purchaser is acquiring the Shares
and the Warrant, and will acquire the shares of Common Stock issuable upon
exercise of the Warrant (the "Warrant Shares" and, together with the Shares and
the Warrant, the "Securities"), for its own account for the purpose of
investment and not with a view to the resale or distribution of any part thereof
and Purchaser has no present intention of selling, granting any participation
in, or otherwise distributing the same.

                  2.4 Disclosure of Information. Purchaser acknowledges that it
has reviewed such information about PRAECIS as it considers necessary or
appropriate for deciding whether to purchase the Securities, and has had an
opportunity to ask questions and receive answers from PRAECIS regarding its
investment in the Securities; provided, however, that such review of such
information shall not be deemed to impair or in any way affect Purchaser's
ability to rely upon PRAECIS' representations, warranties, covenants and other
agreements contained herein.

                  2.5  Investment Experience; Accredited Investor.  Purchaser
acknowledges that it can bear the eco-

                                        5

<PAGE>

nomic risk and complete loss of its investment in the Securities and has such
knowledge and experience in financial or business matters that it is capable of
evaluating the merits and risks of the investment in the Securities. Purchaser
has not been organized solely for the purpose of acquiring the Securities.
Purchaser is an "accredited investor" as defined in Rule 501(a) of Regulation D,
as amended, under the Securities Act (as defined herein).

                  2.6 Restricted Securities. The Securities are characterized as
"restricted securities" under the federal securities laws of the United States
of America inasmuch as they are being or will be acquired from PRAECIS in a
transaction not involving a public offering and that under such laws and
applicable regulations thereunder such securities may be resold without
registration under the United States Securities Act of 1933, as amended (the
"Securities Act"), only in certain limited circumstances. Purchaser is familiar
with Rule 144 promulgated by the U.S. Securities and Exchange Commission (the
"SEC") under the Securities Act ("Rule 144"), as presently in effect, and
understands the resale limitations imposed on the Securities thereby and by
applicable provisions of the Securities Act.

                                        6

<PAGE>

                  2.7 Further Limitations on Disposition. Without in any way
limiting the representations set forth above and subject to the limitations
contained in Article IV hereof, Purchaser further agrees not to make any
disposition of all or any portion of the Securities unless and until:

                  (a) there is then in effect a registration statement under the
Securities Act covering such proposed disposition and such disposition is made
in accordance with such registration statement and any applicable United States
federal or state securities law or regulation; or

                  (b) (i) Purchaser shall have notified PRAECIS of the proposed
disposition and shall have furnished PRAECIS with a detailed statement of the
circumstances surrounding the proposed disposition, and (ii) if reasonably
requested by PRAECIS, Purchaser shall have furnished PRAECIS with an opinion of
counsel reasonably satisfactory to PRAECIS and in form and substance reasonably
satisfactory to PRAECIS that such disposition will not require registration of
such shares under the Securities Act; provided, however, that such prior notice
shall not be required in connection with any permitted transfers

                                        7

<PAGE>

specified in Section 4.1 or in clauses (i) through (vii)
of Section 4.2.

                  2.8  Legends; Stop Transfer Instructions.

                  (a)  Legend and Instructions Applicable to the
Shares. Purchaser hereby acknowledges and agrees that each of the certificates
representing the Shares and the Warrant Shares shall be subject to stop transfer
instructions against the transfer of legended certificates representing the
Shares or the Warrant Shares and shall bear a legend substantially as follows:

       "The shares represented by this certificate have not been
       registered under the Securities Act of 1933, as amended, and
       may not be transferred or otherwise disposed of unless they
       have been registered under such Act or pursuant to an
       exemption from registration under such Act. The sale,
       transfer, pledge or other disposition of the shares
       represented by this certificate is also subject to certain
       limitations set forth in a Stock and Warrant Purchase
       Agreement dated as of May 13, 1997 (the "Purchase Agreement")
       between PRAECIS PHARMACEUTICALS, INC. and Sylamerica, Inc. A
       copy of the Purchase Agreement is on file with the Secretary
       of PRAECIS PHARMACEUTICALS, INC."

                  PRAECIS agrees that, at the request of Purchaser, it will
remove from the certificates representing the Shares (or any relevant portion
thereof) or the Warrant Shares (or any relevant portion thereof) (a) the legend

                                        8

<PAGE>

contemplated by this Section 2.8(a) regarding restrictions under the Securities
Act in the event that outside counsel for Purchaser delivers to PRAECIS an
opinion to the effect that the sale or transfer of such Shares or Warrant Shares
is exempt from the registration requirements of the Securities Act or that such
sale or transfer is no longer restricted by the Securities Act, and (b) the
legend contemplated by this Section 2.8(a) regarding the limitations on transfer
hereunder at the time after which such limitations are no longer applicable or
have been waived in writing by PRAECIS.

                  (b) Legend and Instructions Applicable to Additional Shares.
Purchaser also acknowledges and agrees that any shares of Common Stock (in
addition to the Shares and the Warrant Shares) beneficially owned by Purchaser
or its Affiliates at any time prior to (but not after) the sale, transfer or
other disposition of all (but not less than all) of the Shares and the Warrant
Shares to any third party or parties ("Additional Shares") shall be subject to
similar stop transfer instructions as provided in Section 2.8(a) and shall bear
the following legend:

       "The transfer of the shares repre-
       sented by this certificate is subject
       to certain limitations on transfer

                                        9

<PAGE>

        set forth in a Stock and Warrant
        Purchase Agreement dated as of May
        13, 1997 between PRAECIS PHARMACEUTI-
        CALS, INC. and Sylamerica, Inc.  A
        copy of such agreement is on file
        with the Secretary of PRAECIS
        PHARMACEUTICALS, INC."

Purchaser agrees that if it or any of its Affiliates acquires any Additional
Shares it will promptly surrender to PRAECIS the certificates representing such
Additional Shares. Upon receipt thereof, PRAECIS will cause the aforesaid legend
to be placed on such certificates and such certificates will be promptly
returned to Purchaser. PRAECIS agrees that, at the request of Purchaser, it will
remove such legend from the certificates representing such Additional Shares at
the time after which the limitations on transfer referred to in such legend are
no longer applicable or have been waived in writing by PRAECIS.

                  (c) In addition, PRAECIS agrees that it will instruct its
transfer agent to register the transfer of any Shares, Warrant Shares or
Additional Shares sold by Purchaser or an Affiliate thereof in compliance with
the terms of this Agreement and presented for registration of transfer, provided
that the certificates representing such Shares, Warrant Shares or Additional
Shares are duly endorsed or accompanied by duly executed stock powers and

                                       10

<PAGE>

otherwise in proper form for transfer on the stock trans-
fer books of PRAECIS.

                                   ARTICLE III
                    REPRESENTATIONS AND WARRANTIES OF PRAECIS

                    PRAECIS hereby represents and warrants to

Purchaser as follows:

                  3.1 Due Organization; Authority; Valid Agreement. PRAECIS 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 as a foreign
corporation in each jurisdiction in which such qualification is required, except
where the failure to so qualify would not have a Material Adverse Effect (as
hereinafter defined). PRAECIS has the requisite corporate power and authority to
own or lease the properties which it owns or leases, to carry on its business as
now conducted, to enter into and perform this Agreement and to consummate the
transactions contemplated hereby. The execution, delivery and performance by
PRAECIS of this Agreement and the Warrant, and the consummation by PRAECIS of
the transactions contemplated hereby and thereby, including the issuance and
sale of the Shares and the Warrant pursuant hereto and of the Warrant Shares
pursuant to the

                                       11

<PAGE>

Warrant, have been duly authorized by all requisite corporate action on the part
of PRAECIS. This Agreement and the Warrant have been duly executed and delivered
by PRAECIS and each is a valid and binding agreement of PRAECIS, enforceable
against PRAECIS in accordance with its terms, subject to (i) any applicable
bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium or
other similar laws now or hereafter in effect affecting creditors' rights
generally and (ii) general principles of equity (regardless of whether
enforcement is considered in a proceeding in equity or at law). PRAECIS has no
subsidiaries and does not own or control, directly or indirectly, any capital
stock or other equity interest in any other corporation, association, trust,
partnership, limited liability company, joint venture or other business entity.

                  3.2 No Violation. The execution, delivery and performance by
PRAECIS of this Agreement and the Warrant and the issuance and sale of the
Shares and the Warrant pursuant hereto and of the Warrant Shares pursuant to the
Warrant does not and will not (i) violate or conflict with or result in a breach
of the terms, conditions or provisions of, or constitute (or with notice or
lapse of time, or both, constitute) a default under, (a) PRAECIS'

                                       12

<PAGE>

Amended and Restated Certificate of Incorporation, as amended (the "Charter"),
or Amended and Restated By-Laws (the "By-Laws"), each as currently in effect, or
(b) any mortgage, agreement, instrument, order, judgment, decree, injunction or
decree to which PRAECIS or its properties or assets is bound or subject or (c)
subject to the accuracy of Purchaser's representations contained in Sections
2.3, 2.4, 2.5, 2.6 and 2.7 hereof, any statute, law, rule or regulation to which
PRAECIS or its properties or assets is bound or subject or (ii) subject to the
accuracy of Purchaser's representations contained in Sections 2.3, 2.4, 2.5, 2.6
and 2.7 hereof, require any consent, approval, authorization or permit of, or
filing with, any governmental or regulatory authority, except, in the case of
clause (i)(b), for any violation, conflict, breach or default, which would not
have a material adverse effect on the business, operations or financial
condition of PRAECIS (a "Material Adverse Effect") or any adverse effect on the
legality or validity of the Securities or the rights of Purchaser as a holder of
shares of Common Stock.

                  3.3  Validity of Shares and Warrant Shares.
The Shares and the Warrant Shares, respectively, have
been duly authorized and reserved for issuance and, upon

                                       13

<PAGE>

their issuance in accordance with the terms hereof or of the Warrant,
respectively, will be validly issued, fully paid and nonassessable, and, upon
such issuance, Purchaser will acquire good and valid title to such Shares and
Warrant Shares, respectively, free and clear of any liens, security interests,
options, charges, beneficial interests, claims or encumbrances of any type,
other than those created or caused by Purchaser, and such Shares and Warrant
Shares, respectively, will be free of restrictions on transfer other than
restrictions on transfer pursuant to this Agreement or the Warrant, restrictions
on transfer under applicable state and federal securities laws and restrictions
on transfer created or caused by Purchaser pursuant to agreements with third
parties.

                  3.4 Financial Statements. PRAECIS has delivered to Purchaser
its audited financial statements at December 31, 1996 and 1995 and for the
period from July 16, 1993 through December 31, 1996 and for the years ended
December 31, 1996, 1995 and 1994 (together with the related notes, the
"Financial Statements"). The Financial Statements have been prepared in
accordance with generally accepted accounting principles applied on a consistent
basis throughout the periods indicated, except as may be otherwise stated
therein, and fairly present

                                       14

<PAGE>

the financial position and the results of operations of PRAECIS at the
respective dates and for the respective periods to which they apply. Except as
set forth in the Financial Statements, PRAECIS has no liabilities, contingent or
otherwise, other than (i) liabilities incurred in the ordinary course of
business subsequent to December 31, 1996 and (ii) obligations under contracts
and commitments incurred in the ordinary course of business and not required
under generally accepted accounting principles to be reflected in the Financial
Statements, which, in the case of (i) and (ii) individually or in the aggregate,
have not had a Material Adverse Effect.

                  3.5  Capitalization.

                  (a) On the date hereof, the authorized capital stock of
PRAECIS consists of (i) 4,500,000 shares of Common Stock, of which (A) 455,988
shares have been duly and validly issued, are fully paid and nonassessable and
are presently outstanding, (B) 1,056,091 are reserved for issuance upon
conversion of PRAECIS' Series A Convertible Preferred Stock, par value $.01 per
share (the "Series A Preferred Stock"), including 14,925 shares of Common Stock
issuable upon conversion of the Series A Preferred Stock issuable upon exercise
of the Comdisco Warrants (as defined below), (C) 63,700 shares are reserved for
issu-

                                       15

<PAGE>

ance upon conversion of PRAECIS' Series B Convertible Preferred Stock, par value
$.01 per share (the "Series B Preferred Stock"), (D) 1,052,632 shares are
reserved for issuance upon conversion of PRAECIS' Series C Convertible Preferred
Stock, par value $.01 per share (the "Series C Preferred Stock"), and (E)
962,195 shares are reserved for issuance upon exercise of options that have been
or may be granted under PRAECIS' 1995 Stock Plan, as amended; and (ii) 3,750,000
shares of preferred stock, par value $.01 per share, of PRAECIS ("Preferred
Stock"), of which (A) 1,061,166 shares have been designated as Series A
Preferred Stock, 1,041,166 of which have been duly and validly issued, are fully
paid and nonassessable and are presently outstanding and 14,925 of which are
reserved for issuance upon exercise of the warrants granted to Comdisco, Inc.
pursuant to the Warrant Agreement dated March 29, 1995 between PRAECIS and
Comdisco (the "Comdisco Warrants"), (B) 63,700 shares have been designated as
Series B Preferred Stock, all of which have been duly and validly issued, are
fully paid and nonassessable and are presently outstanding and (C) 1,052,632
shares have been designated as Series C Preferred Stock, all of which have been
duly and validly issued, are fully paid and nonassessable and are presently
outstanding. As of

                                       16

<PAGE>

the date hereof, the conversion price for the Series A Preferred Stock and the
Series B Preferred Stock is $10.085 per share of Common Stock and the conversion
price for Series C Preferred Stock is $19.00 per share of Common Stock. Except
as set forth in this Section 3.5(a), as of the date hereof, PRAECIS has not
reserved any other shares of any class of its capital stock for future issuance.

                  (b) Except as referred to in Section 3.5(a) or as set forth on
Schedule 3.5(b) attached hereto, there are no existing subscriptions, options,
preemptive rights, first offer rights, conversion or exchange rights, warrants,
calls, repurchase or redemption agreements, or other agreements, claims or
commitments of any nature whatsoever obligating PRAECIS to issue, transfer,
deliver, sell, repurchase or redeem shares of capital stock (or securities
convertible into or exchangeable for shares of such capital stock) or obligating
PRAECIS to grant, extend or enter into any such agreement or commitment. Except
for the Amended Stockholders Agreement dated as of April 4, 1996 by and among
PRAECIS and the stockholders of PRAECIS listed on Schedules 1 and 2 thereto, as
amended by Amendment No. 1 dated as of February 13, 1997 (as so amended, the
"Stockholders Agree-

                                       17

<PAGE>

ment"), there are no voting trusts or other agreements or understandings to
which PRAECIS is a party with respect to the voting of the capital stock of
PRAECIS, and PRAECIS is not aware of any other voting trusts, agreements or
understandings with respect to the voting of its capital stock. The issuance
of the Shares or Warrant Shares (assuming the Warrant Shares were issued on
the date hereof) to Purchaser pursuant to the provisions of this Agreement
and the Warrant, respectively, will not be subject to any first refusal or
preemptive rights or any anti-dilution protections given by PRAECIS to any
person or entity (including, without limitation, any stockholder, lender,
warrant-holder, lessor and/or licensor). Without limiting the foregoing, the
transactions contemplated under this Agreement and the Warrant are exempt
from the provisions of Article V of the Stockholders Agreement by virtue of
Section 5.3(viii) of the Stockholders Agreement.

          3.6 Litigation. There is no pending suit, action or litigation,
administrative, arbitration or other proceeding to which PRAECIS is a party
and of which it has received notice, and, to the best knowledge of PRAECIS,
no such suit, action, litigation or proceeding is threatened. There is no
judgment, injunction, decree

                                       18

<PAGE>

or order of any court or other governmental or public authority or agency
outstanding against PRAECIS of which PRAECIS has received written notice.

          3.7 Intellectual Property. PRAECIS owns or has the right to use all
trade names, trademarks, trade dress, patents, trade secrets and proprietary
processes (collectively, "Proprietary Rights") necessary for its business as
presently conducted or as proposed to be conducted pursuant to business plans
previously disclosed in writing to Purchaser without, to PRAECIS' knowledge,
any material conflict with or infringement of the rights of others. PRAECIS
has not received any communication alleging that it has violated, or by
conducting its business as proposed would violate, any of the Proprietary
Rights of any other person or entity. PRAECIS is not aware of any
infringement by any third party on, or any competing claim of the right to
use or own, any of its Proprietary Rights that would, or would be reasonably
likely to, have a Material Adverse Effect.

          3.8 Registration Rights. Except as provided herein and in the
Stockholders Agreement, a copy of which has previously been delivered to
Purchaser, PRAECIS has not granted or agreed to grant any registration
rights, including piggyback rights, to any person or entity.

                                       19

<PAGE>

          3.9 Corporate Documents. The Charter and the By-Laws as in effect
on the Closing Date are attached hereto as Exhibits B and C, respectively.

          3.10 Title to Property and Assets. PRAECIS does not own any real
property. With respect to property leased by PRAECIS, PRAECIS has a valid
leasehold interest in such property pursuant to leases which are in full
force and effect, and PRAECIS is in compliance in all material respects with
the material provisions of such leases. PRAECIS owns or has the right to use
under valid leasehold interests all assets and properties necessary for the
conduct of its business as presently conducted, free and clear of all
mortgages, judgments, claims, liens, security interests, pledges, escrows,
charges or other encumbrances, except for any of the foregoing which were
granted in connection with leasing arrangements or which would not have a
Material Adverse Effect.

          3.11 Tax Returns and Payments. PRAECIS has timely filed all tax
returns and reports as required by law, except where the failure to file any
such tax return or report would not have a Material Adverse Effect. These
returns and reports are true and correct in all material respects. PRAECIS
has paid all taxes and other assessments due prior to the time penalties
would accrue

                                       20

<PAGE>

thereon, except where the failure to pay such taxes or other assessments
would not have a Material Adverse Effect. PRAECIS has not been notified by
any federal or state taxing authority of any audit of the tax returns of
PRAECIS for any tax year, nor has PRAECIS received written notice that any
tax liens have been filed or that any addition to, or deficiency regarding,
taxes has been proposed, asserted or assessed against PRAECIS. PRAECIS has
not granted any extension of the statute of limitations applicable to any tax
return or other tax claim with respect to any taxable year. As used in this
Agreement, the terms "tax" and "taxes" shall, unless otherwise specified,
mean all income taxes (including any tax on or based upon net income, or
gross income, or income as specially defined, or earnings, or profits) and
all gross receipts, sales, use, ad valorem, transfer, franchise, license,
withholding, payroll, employment, excise, severance, stamp, occupation,
premium, property or windfall profits taxes, alternative or add-on minimum
taxes, customs duties or other taxes, fees, assessments or charges of any
kind whatsoever, together with any interest and any penalties, additions to
tax or additional amounts imposed by any taxing authority (domestic or
foreign).

                                       21

<PAGE>

          3.12 Changes. Since December 31, 1996, there has not been:

          (a) any change in the assets, liabilities, financial condition or
operations of PRAECIS except changes in the ordinary course of business which
have not, either individually or in the aggregate, had a Material Adverse
Effect;

          (b) any damage, destruction or loss, whether or not covered by
insurance, causing (or which reasonably could be expected to cause) a
Material Adverse Effect;

          (c) any satisfaction or discharge of any lien, claim or encumbrance
or payment of any obligation by PRAECIS, except in the ordinary course of
business and which has not had a Material Adverse Effect;

          (d) any repurchase of PRAECIS' capital stock (other than the
repurchase of 2,333 shares of Common Stock), or any agreement entered into by
PRAECIS with respect to the repurchase of any of its capital stock (other
than option or severance agreements with current or former employees which
provide for stock repurchases from such employees in certain circumstances);
or

          (e)  any payment of dividends.

          3.13  Contracts.  Except as listed on Schedule 3.13 attached
hereto, as of the date hereof, PRAECIS is

                                       22

<PAGE>

not a party to or bound by any contract or agreement that is material to the
business of PRAECIS. All contracts and agreements listed on Schedule 3.13 are
valid and subsisting and PRAECIS is not in default thereunder nor, to the
best knowledge of PRAECIS, is any other party to any such contract or
agreement in default thereunder, nor to the best knowledge of PRAECIS, does
any condition exist which, with notice or lapse of time or both, would
constitute a default thereunder.

          3.14 Compliance with Laws. PRAECIS currently holds and is in
material compliance with the terms of all licenses, permits and
authorizations necessary for the lawful conduct of the business of PRAECIS as
currently conducted, and has complied with, and is not in violation of, the
applicable statutes, ordinances, rules, regulations, orders or decrees of all
federal, state, local and foreign governmental bodies, agencies and
authorities having or asserting jurisdiction over it or over any part of its
operations or assets, except for any non-compliance, violations or defaults
which individually or in the aggregate would not have a Material Adverse
Effect.

          3.15  Exempt Transaction.  Based in part on the representations and
warranties of Purchaser contained in Sections 2.3, 2.4, 2.5, 2.6 and 2.7 of
this Agreement,

                                       23

<PAGE>

the offer and sale of the Shares and the Warrant pursuant to the terms hereof
are exempt from the registration requirements of the Securities Act.

          3.16 Investment Company Act of 1940. PRAECIS is not an "investment
company" nor an entity "controlled" by an investment company, as such terms
are defined in the Investment Company Act of 1940, as amended.

                                   ARTICLE IV
                    RESTRICTIONS ON TRANSFER OF THE SECURITIES

          4.1 Restrictions on Transfer of Warrant. Without PRAECIS' prior
written consent, neither Purchaser nor any of its Affiliates will, directly
or indirectly, sell, transfer, pledge or otherwise dispose of the Warrant, or
any interest therein, except to an Affiliate of Purchaser (which, for
purposes of this Section 4.1 only, shall include any corporation or other
business organization to which Purchaser shall sell all or substantially all
of its assets or with which it shall be merged or to any third party which
acquires all (but not less than all) of the Shares from Purchaser in a
transaction permitted under the terms hereof) so long as such Affiliate has
agreed in writing with PRAECIS to be bound by this

                                       24

<PAGE>

Agreement and the terms of the Warrant applicable to the Purchaser.

          4.2 Restrictions on Transfer of Common Stock. Without PRAECIS'
prior written consent, neither Purchaser nor any of its Affiliates will,
directly or indirectly, sell, transfer, pledge or otherwise dispose of any
Shares, or any interest therein, except (i) a transfer pursuant to an
effective registration statement under the Securities Act; (ii) a transfer of
Shares complying with Rule 144 as in effect on the date of such transfer and
as applied to Purchaser and its Affiliates (but only a sale pursuant to a
"brokers' transaction" as defined in clauses (i) and (ii) of paragraph (g) of
Rule 144 as in effect on the date hereof); (iii) to PRAECIS, pursuant to a
self-tender offer or otherwise; (iv) to an Affiliate of Purchaser (which, for
purposes of this Section 4.2(iv) only, shall include any corporation or other
business organization to which Purchaser shall sell all or substantially all
of its assets or with which it shall be merged), provided that such Affiliate
agrees in writing with PRAECIS that it will be bound by the provisions of
this Agreement applicable to the Purchaser; (v) to a third party pursuant to
a tender offer recommended by PRAECIS' Board of Directors; (vi) pursuant to
or in

                                       25

<PAGE>

connection with a merger or consolidation in which PRAECIS will be the
acquired corporation, a sale or disposition of all or substantially all of
PRAECIS' assets or a plan of liquidation or dissolution of PRAECIS, which, in
any such case is approved by the stockholders of PRAECIS; (vii) pursuant to a
bona fide pledge of the Shares to secure indebtedness for borrowed money (and
not to circumvent the provisions of this Article IV) in which the pledgee
agrees in writing that, upon any transfer of the Shares to such pledgee, such
Shares shall remain subject to the restrictions set forth in this Agreement;
or (viii) in other bona fide sales for cash made to third parties pursuant to
an exemption from the registration requirements of the Securities Act if,
prior to any such sale, PRAECIS shall have failed (A) to unconditionally
agree in writing, within 20 days after PRAECIS' receipt of written notice
from Purchaser of the proposed sale, to purchase for cash the Shares to be
included in such sale at the same cash price offered by the third-party
purchaser, and (B) to conclude such purchase within 45 days after PRAECIS'
receipt of such written notice from Purchaser, provided however, that the
provisions of this clause (viii) shall be applicable only with respect to
sales and transfers made from and after

                                       26

<PAGE>

the first anniversary of the date hereof and only if each of the conditions
set forth in clauses 1 and 2 below are satisfied: (1) Purchaser has no actual
knowledge (without any independent duty of inquiry) that such third party
purchaser beneficially owns or, after giving effect to such proposed sale
would beneficially own, more than 5% of the then outstanding shares of Common
Stock and (2) such third party purchaser agrees in writing with PRAECIS that
it will be bound by the provisions of this Agreement applicable to the
Purchaser or any Affiliate thereof. Subject to the last sentence of this
Section 4.2, for purposes of this Article IV, the term "Shares" shall include
all shares of Common Stock beneficially owned by Purchaser or any of its
Affiliates, including without limitation any Warrant Shares and any
Additional Shares. Notwithstanding anything to the contrary in the foregoing,
with respect to any person or entity bound by this Agreement, the transfer
restrictions set forth in this Article IV shall cease to apply with respect
to such person or entity at the time such person or entity does not
beneficially own any (i) Shares (excluding for this purpose, any Additional
Shares which may still be owned by such person or entity) or (ii) Warrant
Shares.

                                       27

<PAGE>

                                    ARTICLE V

                                   STANDSTILL

          From the date hereof until the fifth anniversary after the date the
Common Stock is first Publicly Traded (as defined in the Charter as in effect
on the date hereof), without PRAECIS' prior written consent, neither
Purchaser nor any of its Affiliates will, directly or indirectly:

               (a) acquire, offer or propose to acquire, or agree to acquire,
by purchase or otherwise, any Voting Securities, or direct or indirect rights
or options to acquire (through purchase, exchange, conversion or otherwise),
any Voting Securities (as defined herein) if, after giving effect to any such
acquisition, Purchaser, alone or together with its Affiliates, would
beneficially own Voting Securities representing more than 8.2% of the voting
power of all then outstanding Voting Securities, provided, however, that
notwithstanding anything to the contrary contained in this Agreement, the
foregoing 8.2% limitation shall not be deemed to be violated if the
percentage of the voting power of all outstanding Voting Securities
represented by Voting Securities beneficially owned by Purchaser and its
Affiliates is increased as a result of a decrease in the number of
outstanding Voting

                                       28

<PAGE>

Securities caused by a repurchase of securities by PRAECIS or any other
action taken by PRAECIS;

               (b)  make, or in any way participate in, any "solicitation" of
"proxies" to vote (as such terms are used in the proxy rules of the SEC under
the Securities Exchange Act of 1934, as amended (the "Exchange Act")), seek
to advise, encourage or influence any person or entity with respect to the
voting of any Voting Securities, initiate, propose or otherwise solicit
stockholders of PRAECIS for the approval of one or more stockholder proposals
or induce or attempt to induce any other person to initiate any stockholder
proposal; provided, however, that Purchaser shall not, in any event, be
deemed to "solicit" or to be a participant in a "solicitation" for purposes
of this subparagraph (b) by reason of the exercise by Purchaser or its
Affiliates of voting rights with respect to any Voting Securities
beneficially owned by Purchaser or its Affiliates;

               (c) make any proposal, whether written or oral, to the Board
of Directors of PRAECIS, any director or officer of PRAECIS, or make any
public announcement concerning such a proposal, with respect to a tender
offer for any Voting Securities, a merger or other business combination, sale
or transfer of assets, liquidation

                                       29

<PAGE>

or other extraordinary corporate transaction involving PRAECIS;

               (d) form, join or in any way participate in a "group" (within
the meaning of Section 13(d)(3) of the Exchange Act) with respect to any
Voting Securities;

               (e) deposit any Voting Securities beneficially owned by
Purchaser or its Affiliates into a voting trust or subject any such Voting
Securities to any arrangement or agreement with respect to the voting of any
such Voting Securities or any agreement having similar effect, other than a
trust or similar arrangement to which only Purchaser and its Affiliates are
parties;

               (f)  execute any written stockholder con-sent with respect to
any Voting Securities;

               (g)  otherwise act, alone or in concert with others, to seek
to affect or influence the control in any material respect of the management,
policies or Board of Directors of PRAECIS, or make any public statement with
respect thereto;

               (h)  knowingly and intentionally sell or otherwise transfer
any Shares to any "person" (as defined in Section 13 (d)(3) of the Securities
Exchange Act of 1934) or group (within the meaning of Rule 13d-5(b)(1) under
the Securities Exchange Act of 1934) as to which

                                       30

<PAGE>

Purchaser has received notice from PRAECIS of such person's or group's
intention to seek to take, assist or participate in any of the actions
described in clauses (b) through (g) above;

               (i) in any way participate in, encourage, assist or otherwise
induce any person (as that term is used in Section 13(d)(3) of the Exchange
Act) to take, any action prohibited by or inconsistent with the foregoing; or

               (j)  take any other action inconsistent with the foregoing,
provided that seeking any waiver from PRAECIS or any amendment of any
covenant or agreement of or restriction on Purchaser or its Affiliates
contained in this Agreement in a manner which is not calculated or reasonably
likely to be disseminated to the public shall not be deemed "action
inconsistent with the foregoing".

          For the purposes of this Agreement, "Voting Securities" shall mean
all outstanding securities of PRAECIS entitled to vote generally for the
election of directors, including, without limitation, the Common Stock and
"beneficial ownership" of any securities shall be determined pursuant to Rule
13d-3 of the Exchange Act.

                                       31

<PAGE>

                                   ARTICLE VI

                               REGISTRATION RIGHTS

          6.1 Piggyback Registration. (a) If at any time after the
consummation by PRAECIS of an initial public offering of Common Stock PRAECIS
proposes to register (including without limitation any registration effected
by PRAECIS pursuant to Section 6.2 of the Stockholders Agreement) any of its
authorized but unissued Common Stock under the Securities Act on Forms S-1,
S-2, S-3, SB-1, SB-2 or any other registration form at the time in effect on
which Registrable Securities (as defined herein) could be registered for sale
by Purchaser (other than a registration in connection with an acquisition of
or merger with another entity or the sale of shares to employees, consultants
or directors of PRAECIS pursuant to employee stock option, stock purchase or
other employee benefit plans, provided that the only securities covered by
such registration are the securities to be issued as part of such acquisition
or merger or the securities to be sold to such employees, consultants or
directors), PRAECIS shall on each such occasion give written notice to
Purchaser of its intention so to do, describing such Common Stock to be
registered and specifying the form and manner and the other relevant

                                       32

<PAGE>

facts involved in such proposed registration (including, without limitation,
(x) whether or not such proposed registered offering will be an underwritten
offering (an "Underwritten Offering") and, if so, the identity of the
investment banker or bankers that shall manage the offering (the "Managing
Underwriter") and whether such offering will be pursuant to a "best efforts"
or "firm commitment" underwriting and (y) the price (net of any underwriting
commissions, discounts and the like) at which the Registrable Securities, if
any, are reasonably expected to be sold) if such disclosure is acceptable to
the Managing Underwriter. Upon the written request of Purchaser delivered to
PRAECIS within 30 calendar days after the receipt of any such notice (which
request shall specify the Registrable Securities intended to be disposed of
by Purchaser and the intended method of disposition thereof), PRAECIS will
use its reasonable best efforts to effect the registration under the
Securities Act of all of the Registrable Securities that PRAECIS has been so
requested to register; provided, however, that:

                           (i) If, at any time after giving such written notice
         of its intention to register any securities and prior to the effective
         date of the registration statement filed

                                       33

<PAGE>

         in connection with such registration, PRAECIS shall determine for any
         reason not to register such securities, PRAECIS may, at its election,
         give written notice of such determination to Purchaser and thereupon
         shall be relieved of its obligation to register any Registrable
         Securities in connection with such registration (but not from its
         obligation to pay the Registration Expenses (as defined herein) in
         connection therewith); and

                           (ii) If such registration involves an Underwritten
         Offering, Purchaser must sell its Registrable Securities to the
         underwriters selected by PRAECIS on the same terms and conditions as
         apply to PRAECIS.

               (b) The Registration Expenses incurred in connection with each
registration of Registrable Securities requested pursuant to this Section 6.1
shall be paid by PRAECIS.

               (c) If a registration pursuant to this Section 6.1 involves an
Underwritten Offering and the Managing Underwriter advises PRAECIS that, in
its opinion, the number of shares proposed to be included in such
registration should be limited due to market conditions,

                                       34

<PAGE>

then PRAECIS will include in such registration to the extent of the number
which PRAECIS is so advised can be sold in such offering (i) first, the
securities PRAECIS proposes to sell (if any) and (ii) second, the number of
Registrable Securities and shares of Common Stock held by stockholders of
PRAECIS other than Purchaser requested to be included in such registration;
provided, however, that if a greater number of Registrable Securities and
other shares proposed to be offered by other stockholders of PRAECIS are
offered for inclusion in the proposed underwriting than in the opinion of the
Managing Underwriter proposing to underwrite securities to be sold by PRAECIS
(if any) can be accommodated without adversely affecting the proposed
underwriting, PRAECIS may elect to reduce prorata (based upon the amount of
shares owned by stockholders who have requested to have shares which have
registration rights to be included in the proposed underwriting) the amount
of all securities (including shares of Registrable Securities) proposed to be
offered in the underwriting for the accounts of all persons other than
PRAECIS to a number deemed satisfactory by the Managing Underwriter.

               (d)  In connection with any Underwritten Offering with respect
to which Purchaser shall have

                                       35

<PAGE>

requested registration pursuant to subsection 6.1(a), PRAECIS shall have the
right to select the Managing Underwriter with respect to the offering.

          6.2 Registration Procedures. (a) If and whenever PRAECIS is
required to use its reasonable best efforts to effect the registration of any
Registrable Securities under the Securities Act as provided in Section 6.1,
PRAECIS will, as expeditiously as possible:

                           (i) After a registration statement with respect to
         Registrable Securities is filed with the SEC, prepare and file with the
         SEC such amendments (including post-effective amendments) and
         supplements to such registration statement and the prospectus used in
         connection therewith as may be necessary to keep such registration
         statement effective for a period not in excess of 90 days and to comply
         with the provisions of the Securities Act with respect to the
         disposition of all securities covered by such registration statement
         during such period in accordance with the intended methods of
         disposition by Purchaser set forth in such registration statement,
         provided, however, that PRAECIS may discontinue any regis-

                                       36

<PAGE>

         tration of its securities that is being effected pursuant to Section
         6.1 at any time prior to the effective date of the registration state-
         ment relating thereto;

                           (ii) Furnish to Purchaser and to each underwriter, if
         any, of such Registrable Securities, such number of copies of a
         prospectus and preliminary prospectus for delivery in conformity with
         the requirements of the Securities Act, and such other documents, as
         such person may reasonably request, in order to facilitate the public
         sale or other disposition of the Registrable Securities;

                           (iii) Use its best efforts to cause such Registrable
         Securities covered by such registration statement to be registered with
         or approved by such other governmental agencies or authorities as may
         be reasonably necessary to enable Purchaser to consummate the
         disposition of such Registrable Securities;

                           (iv) Immediately notify Purchaser, at any time when a
         prospectus relating thereto is required to be delivered under the
         Securities Act within the appropriate period

                                       37

<PAGE>

         mentioned in Section 6.2(a)(i), if PRAECIS becomes aware that the
         prospectus included in such registration statement, as then in effect,
         includes an untrue statement of a material fact or omits to state any
         material fact required to be stated therein or necessary to make the
         statements therein, in light of the circumstances under which they were
         made, not misleading, and at the request of Purchaser, deliver a
         reasonable number of copies of an amended or supplemental prospectus as
         may be necessary so that, as thereafter delivered to the purchasers of
         such Registrable Securities, such prospectus shall not include an
         untrue statement of a material fact or omit to state a material fact
         required to be stated therein or necessary to make the statements
         therein, in light of the circumstances under which they were made, not
         misleading;

                           (v) Otherwise use its best efforts to comply with all
         applicable rules and regulations of the SEC and make generally
         available to Purchaser, in each case as soon as practicable, but not
         later than 45 calendar

                                       38

<PAGE>

         days after the close of the period covered thereby (90 calendar days in
         case the period covered corresponds to a fiscal year of PRAECIS), an
         earnings statement of PRAECIS which will satisfy the provisions of
         subsection 11(a) of the Securities Act;

                           (vi) In the event the offering is an Underwritten
         Offering, use its best efforts to obtain a "cold comfort" letter from
         the independent public accountants for PRAECIS in customary form and
         covering such matters as are customarily covered by such letters; and

                           (vii) Execute and deliver all instruments and
         documents (including in an Underwritten Offering an underwriting
         agreement in customary form) and take such other actions and obtain
         such certificates and opinions as are customary in underwritten public
         offerings.

               (b)  Purchaser will, upon receipt of any notice from PRAECIS
of the happening of any event of the kind described in subsection 6.2(a)(iv),
forthwith discontinue disposition of the Registrable Securities pursuant to
the registration statement covering such Registrable Securities until
Purchaser's receipt of the copies of

                                       39

<PAGE>

 the supplemented or amended prospectus contemplated by Section 6.2(a)(iv).

               (c) If a registration undertaken by PRAECIS involves an
Underwritten Offering, Purchaser, whether or not Purchaser's Registrable
Securities are included in such registration, will, if requested by the
Managing Underwriter, enter into an agreement not to effect any public sale
or distribution, including any sale pursuant to Rule 144 under the Securities
Act, of any Registrable Securities, or of any security convertible into or
exchangeable or exercisable for any Registrable Securities (other than as
part of such Underwritten Offering), without the consent of the Managing
Underwriter, during a period commencing on the effective date of such
registration and ending a number of calendar days thereafter not exceeding
180 as the Board of Directors of PRAECIS and the Managing Underwriter shall
reasonably determine is required to effect a successful offering.

               (d) If a registration pursuant to Section 6.1 involves an
Underwritten Offering, PRAECIS agrees, if so required by the Managing
Underwriter, not to effect any public sale or distribution of any of its
equity securities or securities convertible into or exchangeable or
exercisable for any of such equity securities during a

                                       40

<PAGE>

period commencing on the effective date of such registration and ending not
more than 180 calendar days thereafter, except for such Underwritten Offering
or in connection with a stock option plan, stock purchase plan, savings or
similar plan, or an acquisition, merger or exchange offer.

          6.3 Indemnification. (a) In the event of any registration of any
securities of PRAECIS under the Securities Act, PRAECIS will, and hereby
does, indemnify and hold harmless Purchaser, if Purchaser has any Registrable
Securities covered by such registration statement, its directors and
officers, each other person who participates as an underwriter in the
offering or sale of such securities and each other person, if any, who
controls Purchaser or any such underwriter within the meaning of the
Securities Act against any losses, claims, damages or liabilities, joint or
several, to which Purchaser or any such director or officer or underwriter or
controlling person may become subject under the Securities Act, state
securities or blue sky laws, or otherwise, insofar as such losses, claims,
damages or liabilities (or actions or proceedings, whether commenced or
threatened, in respect thereof) arise out of or are based upon any untrue
statement or alleged untrue statement of any

                                       41

<PAGE>

material fact contained in any registration statement under which such
securities were registered under the Securities Act, any preliminary
prospectus, final prospectus or summary prospectus contained therein, any
amendment or supplement thereto, or any document filed in connection
therewith or in connection with any qualification pursuant to Section
6.2(a)(iii), or any omission or alleged omission to state therein a material
fact required to be stated therein or necessary to make the statements
therein (in the case of a prospectus, in light of the circumstances under
which they were made) not misleading, and PRAECIS will reimburse Purchaser
and each such director, officer, underwriter and controlling person for any
legal or any other expenses reasonably incurred by them in connection with
investigating or defending any such loss, claim, liability, action or
proceeding; provided that PRAECIS shall not be liable with respect to
Purchaser in any such case to the extent that any such loss, claim, damage,
liability (or action or preceding in respect thereof) or expense arises out
of or is based upon an untrue statement or omission made in such registration
statement, any such preliminary prospectus, final prospectus, summary
prospectus, amendment or supplement, or any document incident thereto or inci-

                                       42

<PAGE>

dent to registration or qualification of any Registrable Securities pursuant
to Section 6.2(a)(iii), in reliance upon and in conformity with written
information furnished to PRAECIS by Purchaser specifically for use in the
preparation thereof; and provided, further, that PRAECIS shall not be liable
to any person who participates as an underwriter, in the offering or sale of
Registrable Securities or to any other person, if any, who controls such
underwriter within the meaning of the Securities Act, in any such case to the
extent that any such loss, claim, damage, liability (or action or proceeding
in respect thereof) or expense arises out of such person's failure to send or
give a copy of the final prospectus, as the same may be then supplemented or
amended, to the person asserting an untrue statement or alleged untrue
statement or omission or alleged omission at or prior to the written
confirmation of the sale of Registrable Securities to such person if such
statement or omission was corrected in such final prospectus. Such indemnity
shall remain in full force and effect regardless of any investigation made by
or on behalf of Purchaser or any such director, officer, underwriter or
controlling person and shall survive the transfer of such securities by
Purchaser.

                                       43

<PAGE>

               (b) PRAECIS may require, as a condition to including any
Registrable Securities in any registration statement filed pursuant to
Section 6.2, that PRAECIS shall have received an undertaking satisfactory to
it from Purchaser, to indemnify and hold harmless (in the same manner and to
the same extent as set forth in subsection (a) of this Section 6.3) PRAECIS,
each director of PRAECIS, each officer of PRAECIS and each other person, if
any, who controls PRAECIS within the meaning of the Securities Act, with
respect to any statement in or omission from such registration statement, any
preliminary prospectus, final prospectus or summary prospectus contained
therein, or any amendment or supplement thereto, or any qualification
pursuant to Section 6.2(a)(iii), if such statement or omission was made in
reliance upon and in conformity with written information furnished to PRAECIS
by Purchaser specifically for use in the preparation of such registration
statement, preliminary prospectus, final prospectus, summary prospectus,
amendment or supplement. Such indemnity shall remain in full force and
effect, regardless of any investigation made by or on behalf of PRAECIS or
any such director, officer or controlling person and shall survive the
transfer of such securities by Purchaser. In no event shall the liability

                                       44

<PAGE>

of Purchaser hereunder be greater in amount than the dollar amount of the
proceeds received by Purchaser upon the sale of the Registrable Securities
giving rise to such indemnification obligation.

               (c) Promptly after receipt by an indemnified party of notice
of the commencement of any action or proceeding involving a claim referred to
in the preceding subsections of this Section 6.3, such indemnified party
will, if a claim in respect thereof is to be made against an indemnifying
party, give written notice to the latter of the commencement of such action,
provided that the failure of any indemnified party to give notice as provided
herein shall not relieve the indemnifying party of its obligations under the
preceding subsections of this Section 6.3, except to the extent that the
indemnifying party is actually prejudiced by such failure to give notice. In
case any such action is brought against an indemnified party, unless in such
indemnified party's reasonable judgment a conflict of interest between such
indemnified and indemnifying parties exists in respect of such claim, the
indemnifying party shall be entitled to participate in and to assume the
defense thereof, jointly with any other indemnifying party similarly
notified, to the extent that the indemnifying party may wish, with

                                       45

<PAGE>

counsel reasonably satisfactory to such indemnified party, and after notice
from the indemnifying party to such indemnified party of its election so to
assume the defense thereof, the indemnifying party shall not be liable to
such indemnified party for any legal or other expenses subsequently incurred
by the latter in connection with the defense thereof other than reasonable
costs of investigation. The indemnified party shall have the right to employ
its own counsel in any such action, but the fees and expenses of such counsel
shall be at the expense of such indemnified party, when and as incurred,
unless (i) the employment of counsel by such indemnified party has been
authorized by the indemnifying parties, (ii) the indemnified party shall have
reasonably concluded that there is a conflict of interest between the
indemnifying parties and the indemnified party in the conduct of the defense
of such action (in which case the indemnifying parties shall not have the
right to direct the defense of such action on behalf of the indemnified
party) or (iii) the indemnifying parties shall not in fact have employed
counsel to assume the defense of such action. No indemnifying party shall,
without the consent of the indemnified party, consent to entry of any
judgment or enter into any settlement of any such action

                                       46

<PAGE>

which does not include as an unconditional term thereof the giving by the
claimant or plaintiff to such indemnified party of a release from all
liability in respect to such claim or litigation. No indemnified party shall
consent to entry of any judgment or enter into any settlement of any such
action the defense of which has been assumed by an indemnifying party or for
which an indemnifying party may have liability hereunder without the consent
of such indemnifying party.

          6.4 Contribution. In order to provide for just and equitable
contribution in circumstances under which the indemnity contemplated by
Section 6.3 is for any reason not available, the parties entitled to
indemnification by the terms hereof shall contribute to the aggregate losses,
liabilities, claims, damages and expenses of the nature contemplated by such
indemnity incurred by PRAECIS, Purchaser and one or more of the underwriters,
except to the extent that contribution is not permitted under Section 11(f)
of the Securities Act. In determining the amount of contribution to which the
respective parties shall be entitled, there shall be considered the relative
benefits received by each party from the offering of the securities,
including the Registrable Securities, (taking into account the portion of

                                       47

<PAGE>

the proceeds of the offering realized by each), the parties' relative
knowledge and access to information concerning the matter with respect to
which the claim was asserted, the opportunity to correct and prevent any
statement or omission and any other equitable considerations appropriate
under the circumstances. PRAECIS and Purchaser agree with each other that
Purchaser shall not be required to contribute any amount in excess of the
amount Purchaser would have been required to pay to an indemnified party if
the indemnity under subsection 6.3(b) was available. PRAECIS and Purchaser
agree with each other and the underwriters of the Registrable Securities, if
required by such underwriters, that it would not be equitable if the amount
of such contribution were determined by pro-rata or per capita allocation
(even if the underwriters were treated as one entity for such purpose) or for
the underwriters' portion of such contribution to exceed the percentage that
the underwriting discount bears to the offering price of the Registrable
Securities. For purposes of this Section 6.4, each person, if any, who
controls an underwriter within the meaning of Section 15 of the Securities
Act, shall have the same rights to contribution as such underwriter, and each
director and each officer of PRAECIS who signed the

                                       48

<PAGE>

registration statement, and each person, if any, who controls PRAECIS or
Purchaser within the meaning of Section 15 of the Securities Act shall have
the same rights to contribution as PRAECIS or Purchaser, as the case may be.

          6.5 Rule 144. If PRAECIS shall have filed a registration statement
pursuant to the requirements of Section 12 of the Exchange Act or a
registration statement pursuant to the requirements of the Securities Act,
PRAECIS covenants that it will file the reports required to be filed by it
under the Securities Act and the Exchange Act and the rules and regulations
adopted by the SEC thereunder (or, if PRAECIS is not required to file such
reports, it will, upon the request of Purchaser, make publicly available
other information), and it will take such further action as Purchaser may
reasonably request, all to the extent required from time to time to enable
Purchaser to sell shares of Registrable Securities, subject to the applicable
restrictions on transfers contained herein, without registration under the
Securities Act within the limitation of the exemptions provided by (i) Rule
144, as such rule may be amended from time to time, or (ii) any similar rule
or regulation hereafter adopted by the SEC. Upon the request of Purchaser,

                                       49

<PAGE>

PRAECIS will deliver to Purchaser a written statement as to whether it has
complied with such requirements.

          6.6 Definitions. (a) "Registrable Securities" shall mean (i) the
Shares, (ii) the Warrant Shares, and (iii) any equity securities issued in
exchange or substitution for, or in payment of dividends on, any such shares
referred to in clauses (i) and (ii) of this definition. Any particular
Registrable Securities shall cease to be Registrable Securities when either
(i) a registration statement with respect to the sale of such securities
shall have become effective under the Securities Act and such securities
shall have been disposed of under such registration statement, (ii) such
securities shall have been transferred pursuant to Rule 144, (iii) such
securities shall have been otherwise transferred or disposed of, and new
certificates therefor not bearing a legend restricting further transfer
thereof under the Securities Act shall have been delivered by PRAECIS and
subsequent transfer or disposition thereof shall not require their
registration or qualification under the Securities Act or any similar state
law then in force, or (iv) such securities shall have ceased to be
outstanding.

               (b)  "Registration Expenses" shall mean any and all
out-of-pocket expenses incident to PRAECIS'

                                       50

<PAGE>

performance of or compliance with this Article VI, including, without
limitation, all SEC, stock exchange or National Association of Securities
Dealers, Inc. ("NASD") registration and filing fees, all fees and expenses of
complying with securities and blue sky laws (including the reasonable fees
and disbursements of underwriters' counsel in connection with blue sky
qualification and NASD filings), all fees and expenses of the transfer agent
and registrar for the Common Stock, all printing expenses, the fees and
disbursements of counsel for PRAECIS and of its independent public
accountants, including the expenses of any special audits and/or "cold
comfort" letters required by or incident to such performance and compliance,
and the reasonable fees and disbursements of one counsel (other than house
counsel) retained by Purchaser, but excluding (a) any allocation of the
personnel or other general overhead expenses of PRAECIS or other expenses for
the preparation of financial statements or other data normally prepared by
PRAECIS in the ordinary course of its business, which shall be borne by
PRAECIS in all cases, and (b) underwriting discounts and commissions and
applicable transfer and documentary stamp taxes, if any, which shall be borne
by Purchaser in all cases.

                                       51

<PAGE>

                                   ARTICLE VII
                       INDEMNIFICATION; CERTAIN COVENANTS

                   7.1 Indemnification. (a) PRAECIS agrees to
indemnify, defend and hold Purchaser harmless from and against any demand,
liability, loss, deficiency, damage, cost or expense (including reasonable legal
and accounting fees and expenses but excluding consequential damages) to
Purchaser arising out of any breach of any representation or warranty of PRAECIS
or any nonfulfillment of any covenant or agreement of PRAECIS contained herein.
Purchaser shall not be entitled to indemnification with respect to any claim
under the foregoing provisions of this Section 7.1(a) as to which notice shall
not have been given by Purchaser to PRAECIS (i) with respect to indemnification
for claims arising out of any such nonfulfillment of any such covenant or
agreement, within two years of the date of occurrence of such nonfulfillment or
(ii) with respect to indemnification for claims arising out of breach of the
representations and warranties of PRAECIS contained in Article III, on or before
the applicable date specified in Section 9.1(b) below.

                  (b) Purchaser agrees to indemnify, defend and hold PRAECIS
harmless from and against any demand, lia-

                                       52

<PAGE>

bility, loss, deficiency, damage, cost or expense (including reasonable legal
and accounting fees and expenses but excluding consequential damages) to
PRAECIS arising out of any breach of any representation or warranty of
Purchaser or any nonfulfillment of any covenant or agreement of Purchaser
contained herein. PRAECIS shall not be entitled to indemnification with
respect to any claim under the foregoing provisions of this Section 7.1(b) as
to which notice shall not have been given by PRAECIS to Purchaser (i) with
respect to indemnification for claims arising out of any such nonfulfillment
of any such covenant or agreement, within two years of the date of occurrence
of such nonfulfillment or (ii) with respect to indemnification for claims
arising out of breach of the representations and warranties of Purchaser
contained in Article II, on or before the applicable date specified in
Section 9.1(a) below.

                  (c) Promptly after receipt by an indemnified party under
this Agreement of notice of any claim or the commencement of any action, the
indemnified party shall, if a claim in respect thereof is to be made against
the indemnifying party hereunder, notify the indemnifying party in writing of
such claim or the commencement of such action. Following such notice, the
indemnifying

                                       53

<PAGE>

party shall be entitled to participate therein and, to the extent it wishes,
to assume the defense thereof with counsel reasonably satisfactory to the
indemnified party. After notice from the indemnifying party to the
indemnified party of its election to assume the defense of any such claim or
action, the indemnifying party shall not be liable to the indemnified party
under this Section 7.1 for any legal fees subsequently incurred by the
indemnified party in connection with the defense thereof; provided, however,
that the indemnified party shall have the right to employ separate counsel in
connection with any such claim or action if it shall have received an opinion
of counsel reasonably acceptable to the indemnifying party that, in the
opinion of such counsel, separate representation of the indemnified party is
appropriate in light of conflicting interests or other factors, and, in such
event, the reasonable fees and expenses of one such separate counsel shall be
paid by the indemnifying party.

                  7.2 Information to be Furnished. Until such time as PRAECIS
has a class of securities which is Publicly Traded (as defined in the Charter
as in effect on the Closing Date), so long as Purchaser and its Affiliates
beneficially own at least one half of the total number of shares and Common
Stock beneficially owned by

                                       54

<PAGE>

them immediately after giving effect to the Closing (including for this
purpose the Warrant Shares), PRAECIS shall furnish to Purchaser copies of all
information which is provided to the holders of any series of its preferred
stock, including, without limitation, copies of its annual audited and
monthly unaudited financial statements and all annual budgets.

                  7.3 Closing Deliveries. Simultaneously with the execution
and delivery of this Agreement, the following deliveries shall be made:

                    (a) Each party shall deliver to the other party a
certificate of a duly authorized officer of such party certifying as to the
resolutions of the Board of Directors of such party authorizing such party's
execution, delivery and performance of this Agreement and the consummation by
such party of the transactions contemplated hereby.

                    (b) PRAECIS shall cause to be delivered to Purchaser an
opinion from Skadden, Arps, Slate, Meagher & Flom LLP, counsel to PRAECIS, in
form and substance satisfactory to Purchaser.

                    (c) Purchaser shall cause to be delivered to PRAECIS an
opinion from Coudert Brothers, counsel to Purchaser, in form and substance
satisfactory to PRAECIS.

                                       55

<PAGE>

                                  ARTICLE VIII
                                   ENFORCEMENT

                  8.1 Injunctive Relief. Purchaser and PRAECIS acknowledge
that neither party would have an adequate remedy at law for money damages in
the event that any of the covenants or agreements of the other party in this
Agreement were not performed in accordance with its terms. Therefore, the
parties agree that each party shall be entitled to specific enforcement of
such covenants or agreements and to injunctive and other equitable relief in
addition to any other remedy to which such party may be entitled, at law or
in equity.

                  8.2 Consent to Jurisdiction and Venue. Purchaser and
PRAECIS consent to personal jurisdiction in any action arising under this
Agreement brought in any court, federal or state, within the State of New
York of the United States of America having subject matter juris- diction.
The parties further agree that such courts will be proper fora in which to
adjudicate any case or controversy arising hereunder, and that neither of
them will object to such venue or the jurisdiction of such courts. The
parties further agree that only the state and federal courts within the State
of New York shall have jurisdiction over any such case or controversy
arising hereunder.

                                       56

<PAGE>

The parties acknowledge that the agreement of each party to submit to such
venue and jurisdiction is a material inducement to the other party to enter
into this Agreement.

                  8.3 Service of Process. The parties agree that service of
any summons or other legal process in any action arising under this Agreement
may be made, and shall not be objected to if made, in the manner provided in
Section 8.3 hereof.

                                   ARTICLE IX
                                  MISCELLANEOUS

                  9.1 Survival.

                    (a) Except as set forth below, all representations and
warranties of Purchaser set forth herein shall survive consummation of the
purchase and sale of the Shares and the Warrant in accordance with the terms
hereof until eighteen months after the Closing Date. If the Warrant is
exercised by Purchaser, as a condition to each such exercise, Purchaser shall
deliver to PRAECIS a certificate of a duly authorized officer of Purchaser
certifying as to the truth and accuracy of Purchaser's representations and
warranties contained in Sections 2.3, 2.4, 2.5, 2.6 and 2.7 hereunder and, in
such event, such

                                       57

<PAGE>

representations and warranties of Purchaser shall survive for a period of
eighteen months after the date of issuance of such Warrant Shares. For
purposes of the preceding two sentences, the applicable survival period shall
continue during the pendency of any suit, claim or other proceeding brought
in respect of such representations and warranties prior to the relevant
expiration date.

                    (b) All representations and warranties of PRAECIS set
forth herein shall survive consummation of the purchase and sale of the
Shares and the Warrant in accordance with the terms hereof until the date
which is eighteen months after the Closing Date (provided that such survival
period shall continue during the pendency of any suit, claim or other
proceeding brought in respect of such representations and warranties prior to
the termination of such eighteen-month period).

                  9.2 Entire Agreement; No Assignment; Severability. This
Agreement, and the other agreements referred to herein, contain the entire
understanding of the parties with respect to the subject matter hereof and
thereof and may not be amended except by a writing signed by the parties.
Neither this Agreement, nor any rights hereunder, shall be assignable by
either of the parties,

                                       58

<PAGE>

except that Purchaser may assign this Agreement and its rights hereunder to
an Affiliate of Purchaser in connection with a transfer of Shares, the
Warrant or Warrant Shares to such Affiliate in compliance with the applicable
terms of this Agreement and the Warrant. Additionally, in transactions
permitted hereunder, the rights granted to Purchaser under Article VI may be
transferred or succeeded to by a person or entity that acquires substantially
all of the assets of Purchaser or with which Purchaser is merged so long as
such transferee provides PRAECIS with a written undertaking to be bound by
all of the provisions of this Agreement applicable to the Purchaser. After
two years from the date hereof, in transactions permitted hereunder, the
rights granted to Purchaser under Article VI may also be transferred by
Purchaser to any other person or entity that acquires at least 50% of the
Registrable Securities so long as such transferee provides PRAECIS with a
written undertaking to be bound by all of the provisions of this Agreement
applicable to Purchaser. A transferee to whom rights under Article VI are
transferred pursuant to the preceding two sentences may not thereafter
transfer such rights to any other person or entity. This Agreement shall be
binding upon the respective successors of the parties.

                                       59

<PAGE>

In the event that any provision of this Agreement shall be declared
unenforceable by a court of competent jurisdiction, such provision, to the
extent declared unenforceable, shall be stricken and the remainder of this
Agreement shall remain binding on the parties hereto. However, in the event
any such provision shall be declared unenforceable due to its scope, breadth
or duration, then it shall be modified to the scope, breadth or duration
permitted by law and shall continue to be fully enforceable as so modified.

                  9.3 Notices. Any notices and other communications given
pursuant to this Agreement shall be in writing and shall be effective upon
delivery by hand or upon receipt if sent by mail (registered or certified mail,
postage prepaid, return receipt requested) or upon receipt of confirmation of
delivery if sent by telex or telecopy. Notices are to be addressed as follows:

                  If to PRAECIS:

                  PRAECIS PHARMACEUTICALS, INC.
                  One Hampshire Street
                  Cambridge, Massachusetts  02139
                  Attention:  Chief Financial Officer
                  Telephone:  (617) 494-8400
                  Telecopy:   (617) 494-8414

                  with a copy to:

                  Skadden, Arps, Slate, Meagher & Flom LLP
                  One Beacon Street

                                       60

<PAGE>

                  Boston, Massachusetts  02108
                  Attention:  Kent A. Coit, Esq.
                  Telephone:  (617) 573-4800
                  Telecopy:   (617) 573-4822

                  If to Purchaser:

                  Sylamerica, Inc.
                  660 White Plains Road
                  Suite 400
                  Tarrytown, New York  10591
                  Attention:  President
                  Telephone:  (914) 332-0165
                  Facsimile:  (914) 332-0245

                  with a copy to:

                  Synthelabo
                  22 Avenue Galilee
                  92350 Le-Plessis-Robinson
                  France
                  Attention:  Director de Projet-
                              Accords et Licenses
                  Telephone: (011) 331-45-37-90-16
                  Telecopy:  (011) 331-45-37-59-49

                  and to:

                  Coudert Brothers
                  1114 Avenue of the Americas
                  New York, New York  10036-7703
                  Attention:  James Colihan, Esq.
                  Telephone:  (212) 626-4680
                  Facsimile:  (212) 626-4120

or to such other addresses as either PRAECIS or Purchaser shall designate to
the other by notice in writing in accordance with the provisions of this
Section 9.3, provided that notice of a change of address shall be effective
only upon receipt.

                                       61

<PAGE>

                  9.4 Best Efforts. Subject to the terms and conditions of
this Agreement, each of the parties hereby agrees to use all reasonable
efforts to take, or cause to be taken, all action and to do, or cause to be
done, all things necessary, proper or advisable under applicable laws, rules
and regulations to consummate and make effective the transactions
contemplated by this Agreement, including using its best efforts to make all
required filings and to obtain all necessary waivers, consents and approvals.
In case at any time after the execution of this Agreement, further action is
necessary or desirable to carry out the purposes of this Agreement, the
proper officers and directors of each of the parties shall take all such
necessary actions.

                  9.5 Fees and Expenses. Each of the parties hereto shall pay
its own fees and expenses (including the fees of any attorneys, accountants,
investment bankers or others engaged by such party) incurred in connection
with this Agreement and the transactions contemplated hereby, whether or not
such transactions are consummated.

                  9.6 Certain Definitions. As used in this Agreement, (i) the
term "business day" means any day except a Saturday, Sunday or a day on which
banking institutions in the State of New York are obligated by

                                       62

<PAGE>

law, regulation or governmental order to close, (ii) the term "Affiliate"
with respect to a Person means any other Person directly or indirectly,
through one or more intermediaries, controlling, controlled by or under
common control with such Person (for purposes of this definition "control"
(including, with correlative meanings, the terms "controlled by" and "under
common control with"), as used with respect to any Person, shall mean the
possession, directly or indirectly, of the power to direct or cause the
direction of the management and policies of such Person, whether through the
ownership of voting securities, by contract or otherwise) and (iii) the term
"Person" means any individual, corporation, trust, association, partnership,
limited liability company, other business entity or any government, political
subdivision thereof or governmental agency.

                  9.7 Counterparts. This Agreement may be executed in
counterparts, which together shall be considered one and the same agreement
and each of which shall be deemed an original.

                  9.8 Governing Law. This Agreement shall be governed by and
construed in accordance with the laws of the State of Delaware, without
giving effect to the conflicts of law principles thereof.

                                       63

<PAGE>

                  9.9 Headings. The Article and Section headings contained in
this Agreement are for reference purposes only and shall not affect in any
way the meaning or interpretation of this Agreement.

                                       64

<PAGE>

                  IN WITNESS WHEREOF, each of the parties has executed this
Agreement on the date first written above.

                                            PRAECIS PHARMACEUTICALS, INC.

                                            By /s/ Malcolm L. Gefter
                                              --------------------------------
                                              Name:   Malcolm L. Gefter
                                              Title:  Chairman of the Board,
                                                      Chief Executive
                                                      Officer and Treasurer

                                            SYLAMERICA, INC.

                                            By /s/ George Liney
                                              --------------------------------
                                              Name:   George Liney
                                              Title:  V.P. Finance and
                                                      Administration

                                       65

<PAGE>

                                                                Schedule 3.5(b)

Amended Stockholders Agreement dated as of April 4, 1996 by and among the
Company and certain stockholders of the Company, as amended as of February 13,
1997

Master Lease Agreement dated March 29, 1995 by and between the Company and
Comdisco, Inc.

Option Agreements entered into pursuant to the Company's 1995 Stock Plan, as
amended

Letter Agreement dated February 2, 1994 by and between the Company and David
Sharrock

                                       66

<PAGE>

                                                                  Schedule 3.13

Collaboration and License Agreement dated as of August 1, 1996 by and between
the Company and Boehringer Ingelheim International GmbH

License Agreement effective as of October 17, 1996 by and between the Company
and Indiana University Foundation

License Agreement effective as of March 1996 by and among the Company, Dyax
Corp. and Protein Engineering Corporation

Lease dated as of April 28, 1994 by and between the Company and The Charles
Stark Draper Laboratory, Inc.

1995 Stock Plan, as amended

Amended Stockholders Agreement dated as of April 4, 1996 by and among the
Company and certain stockholders of the Company, as amended as of February 13,
1997

Master Lease Agreement dated March 29, 1995 by and between the Company and
Comdisco, Inc.

                                       67

<PAGE>

                                                                     Exhibit A

     This Warrant and any securities acquired upon the exercise of this Warrant
     have not been registered under the Securities Act of 1933, as amended, and
     may not be transferred or otherwise disposed of  unless they have been
     registered under such Act or pursuant to an exemption from registration
     under such Act.  The sale, transfer, pledge or other disposition of this
     Warrant and such securities is also subject to certain limitations set
     forth in a Stock and Warrant Purchase Agreement dated as of May 13, 1997
     (the "Purchase Agreement") between PRAECIS PHARMACEUTICALS, INC. and
     Sylamerica, Inc.  A copy of the Purchase Agreement is on file with the
     Secretary of PRAECIS PHARMACEUTICALS, INC.  This Warrant and such
     securities may be sold, transferred, pledged or otherwise disposed of only
     upon the fulfillment of the conditions specified in the Purchase Agreement
     and this Warrant.

                            PRAECIS PHARMACEUTICALS, INC.

                  Warrant for the Purchase of Shares of Common Stock

                                                                   53,926 Shares

     THIS CERTIFIES that, for value received, Sylamerica, Inc., a Delaware
corporation (including any successor, the "Holder"), is entitled to subscribe
for and purchase from PRAECIS PHARMACEUTICALS, INC., a Delaware corporation
(including any successor, the "Company"), at any time or from time to time
subsequent to May 13, 1997 and prior to 5:00 p.m. on May 13, 2002, Boston,
Massachusetts time (the "Exercise Period"), 53,926 fully paid, validly issued
and nonassessable shares (the "Shares") of the Company's Common Stock, $.01 par
value per share (the "Common Stock"), at a purchase price of $96.6 per Share,
subject to adjustment as provided herein (the "Exercise Price"), upon the terms
and subject to the conditions set forth herein.  This Warrant is the warrant
originally issued to the Holder pursuant to the Stock and Warrant Purchase
Agreement dated as of May 13, 1997 between the Company and the Holder (the
"Purchase Agreement").  As used herein the term "this Warrant" shall mean and

<PAGE>

include any Warrant or Warrants hereafter issued in consequence of the transfer
of this Warrant or the exercise of this Warrant in part.  This Warrant is not
transferable in whole or in part, except to an Affiliate (as defined in Section
4.1 of the Purchase Agreement) of the Holder which has agreed in writing with
the Company to be bound by the Purchase Agreement and the terms hereof.

     1.  This Warrant may be exercised in whole or in part at any time during
the Exercise Period by the surrender of this Warrant (with the Election to
Exercise attached hereto as Exhibit I duly executed) to the Company at its
office at One Hampshire Street, Cambridge, MA 02139, Attention:  Chief Financial
Officer, or such other place as may be designated in writing by the Company,
together with a certified or bank cashier's check payable to the order of the
Company in an amount equal to the Exercise Price multiplied by the number of
Shares covered by such exercise.

     2.  Upon exercise of this Warrant in whole or in part, the Holder shall be
deemed to be the holder of record of the Shares issuable upon such exercise,
notwithstanding that the stock transfer books of the Company shall then be
closed or certificates representing such Shares shall not then have been
physically delivered to the Holder.  As soon as practicable after each such
exercise of this Warrant, but not later than five days from the date of such
exercise, the Company shall issue and deliver to the Holder a certificate or
certificates for the Shares issuable upon such exercise, registered in the name
of the Holder.  The Company shall not be required to issue stock certificates
representing fractions of Shares, but shall, in respect of any final fraction of
a Share, make a payment in cash in an amount equal to the same fraction
(calculated to the nearest 1/100th of a Share) of the closing sale price per
share of the Common Stock on the principal public trading market on which the
Common Stock is then traded, or, if the Common Stock is not then traded on any
public trading market, in an amount determined in good faith by the Company's
Board of Directors.  In case an exercise of this Warrant is in part only,
concurrently with delivering to the Holder the certificate or certificates for
the Shares issuable upon such exercise as provided above, the Company shall
deliver to the Holder a new Warrant of like tenor, calling in the aggregate on
the face thereof for the number of

                                       2
<PAGE>

remaining Shares as to which the Holder is entitled to receive upon any
further exercise of this Warrant.

     3.  The Company shall at all times reserve for issuance and keep available
out of its authorized and unissued shares of Common Stock, solely for the
purpose of providing for the exercise of this Warrant, such number of Shares as
shall from time to time be sufficient therefor.

     4.  (a)  The Exercise Price shall be subject to adjustment from time to
time in case the Company shall (i) declare a dividend or make a distribution on
the outstanding shares of Common Stock, in shares of Common Stock, (ii)
subdivide or reclassify the outstanding shares of Common Stock into a greater
number of shares of Common Stock, or (iii) combine or reclassify the outstanding
shares of Common Stock into a smaller number of shares of Common Stock.  In each
such case, the Exercise Price in effect at the time of the record or effective
date, as the case may be, of such dividend, declaration, distribution,
subdivision, combination or reclassification shall be adjusted so that it shall
equal the price determined by multiplying the Exercise Price by a fraction, the
denominator of which shall be the number of shares of Common Stock outstanding
immediately after giving effect to such action, and the numerator of which shall
be the number of shares of Common Stock outstanding immediately prior to such
action.

         (b)  Whenever the Exercise Price upon exercise of this Warrant is
adjusted pursuant to Paragraph 4(a), the number of Shares issuable upon exercise
of this Warrant shall simultaneously be adjusted by multiplying the number of
Shares initially issuable upon exercise of this Warrant by the initial Exercise
Price in effect on the date hereof and dividing the product so obtained by the
Exercise Price, as adjusted.

         (c)  Whenever there shall be an adjustment as provided in this
Paragraph 4 or in Paragraph 5 below, the Company shall promptly cause written
notice thereof to be sent by registered mail, postage prepaid to the Holder, at
its principal office, which notice shall be accompanied by a certificate setting
forth the Exercise Price after such adjustment and a brief statement of the
facts requiring such adjustment and the computation thereof.

                                       3
<PAGE>

         (d)  All calculations pursuant to this Paragraph 4 shall be made to
the nearest cent or one-hundredth of a Share, as the case may be.

     5.  (a)  In case of any reclassification or change of the shares of Common
Stock issuable upon exercise of this Warrant (other than a change in par value
or from par value to no par value, or as a result of a subdivision or
combination, but including any change in such shares into two or more classes or
series of shares) or in case of any consolidation or merger of the Company into
another corporation or of another corporation into the Company in which the
Company is the surviving corporation and in which there is a reclassification or
change (including change by way of a conversion into the right to receive cash
or other property) of the shares of Common Stock (other than a change in par
value, or from par value to no par value, or as a result of a subdivision or
combination, but including any change in such shares into two or more classes or
series of shares), appropriate  provision shall be made so that the Holder shall
have the right thereafter to receive upon exercise of this Warrant solely the
kind and amount of shares of stock and other securities, property, cash or any
combination thereof receivable upon such reclassification, change, consolidation
or merger by a holder of the number of shares of Common Stock for which this
Warrant could have been exercised immediately prior to such reclassification,
change, consolidation or merger.  Thereafter, appropriate provision (as
determined reasonably and in good faith by the Company's Board of Directors)
shall be made for adjustments which shall be as nearly equivalent as practicable
to the adjustments provided for in Paragraph 4.

         (b)  The above provisions of this Paragraph 5 shall apply to
successive reclassifications and changes of shares of Common Stock and to
successive consolidations or mergers.

     6.  The Company will not, by amendment of its articles of incorporation or
through any reorganization, transfer of assets, consolidation, merger,
dissolution, issue or sale of securities or any other voluntary action, avoid or
seek to avoid the observance or performance of any of the terms of this Warrant.
Without limiting the generality of the foregoing, the Company (a)

                                      4
<PAGE>

will not increase the par value of any shares of stock receivable upon the
exercise of this Warrant above the amount payable therefor upon such
exercise, and (b) will take all such action as may be necessary or
appropriate in order that the Company may validly and legally issue fully
paid and nonassessable shares of stock upon the exercise of this Warrant from
time to time outstanding.

     7.  The Shares or other securities issued upon exercise of this Warrant
shall be subject to a stop-transfer order and the certificate or certificates
evidencing any such Shares or securities shall bear the following legend:

         "The shares represented by this certificate have not been
         registered under the Securities Act of 1933, as amended, and
         may not be transferred or otherwise disposed of unless they
         have been registered under such Act or pursuant to an
         exemption from registration under such Act.  The sale,
         transfer, pledge or other disposition of the shares
         represented by this certificate is also subject to certain
         limitations set forth in a Stock and Warrant Purchase
         Agreement dated as of May 13, 1997 (the "Purchase
         Agreement") between PRAECIS PHARMACEUTICALS, INC. and
         Sylamerica, Inc.  A copy of the Purchase Agreement is on
         file with the Secretary of PRAECIS PHARMACEUTICALS, INC."

     8.  Upon receipt of evidence or sworn statement satisfactory to the
Company of the loss, theft, destruction or mutilation of this Warrant and upon
surrender and cancellation of this Warrant if mutilated, and upon reimbursement
of the Company's reasonable incidental expenses, the Company shall execute and
deliver to the Holder thereof a new Warrant of like date, tenor and denomination

     9.  This Warrant shall be governed by and construed in accordance with the
laws of the State of Delaware, without regard to the conflicts of laws
principles thereof.

                                       5
<PAGE>

Dated: May 13, 1997

                         PRAECIS PHARMACEUTICALS, INC.

                         By
                            ----------------------------------------------
                            Name:
                            Title:

Attest:

--------------------------
Name:
Title:

                                       6

<PAGE>

                                      Exhibit I

To:  PRAECIS PHARMACEUTICALS, INC.

                                 ELECTION TO EXERCISE

          The undersigned hereby exercises its right to subscribe for and
purchase from PRAECIS PHARMACEUTICALS, INC., _________ fully paid, validly
issued and nonassessable shares of Common Stock covered by the within Warrant
and tenders payment herewith in the amount of $________ in accordance with the
terms thereof, and requests that certificates for such shares be issued in the
name of, and delivered to:

                    Sylamerica, Inc.
                    660 White Plains Road
                    Suite 400
                    Tarrytown, New York
                    TIN:

Date:                         SYLAMERICA, INC.
     -------------------------

                              By
                                 ----------------------------
                                 Name:
                                 Title:

                                       7

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