<PAGE>

                                                                  Exhibit 10.4

                   AMENDED AND RESTATED STOCKHOLDERS AGREEMENT


                  AMENDED AND RESTATED STOCKHOLDERS AGREEMENT dated as of
April 30, 1998 (the "Agreement") by and among PRAECIS PHARMACEUTICALS
INCORPORATED, a Delaware corporation (the "Company"), Malcolm L. Gefter ("Dr.
Gefter"), Ethan R. Signer ("Dr. Signer"), A. Donny Strosberg ("Dr.
Strosberg"), the other current Stockholders (as defined herein) as set forth
on Schedule 1 attached hereto (the "Existing Investors"), the additional
parties as set forth on Schedule 2 attached hereto (the "New Investors"), and
any additional Persons (as defined herein) who shall become parties to this
Agreement as Stockholders in accordance with the terms hereof.

                  WHEREAS, the Company, Dr. Gefter, Dr. Signer, Dr. Strosberg
and certain of the Existing Investors were parties to a Stockholders
Agreement dated as of December 23, 1993, as supplemented as of March 3, 1994
and amended as of December 2, 1994 (the "Original Stockholders Agreement");
and

                  WHEREAS, pursuant to Section 9.2 of the Original
Stockholders Agreement, the Original Stockholders Agreement was amended by
the Amended Stockholders Agreement, dated as of April 4, 1996, as amended by
Amendment No. 1 thereto dated as of February 13, 1997 and Amendment No. 2
thereto dated as of June 9, 1997 (such agreement as so amended, the "Amended
Stockholders Agreement"); and

                  WHEREAS, pursuant to Section 9.2 of the Amended
Stockholders Agreement, the Amended Stockholders Agreement may be amended by
a written instrument duly executed by the Company and the holders of a
majority of the then outstanding shares of the Company's common stock, par
value $.01 per share (the "Common Stock"), and 66 2/3% of the shares of
Common Stock issued or issuable upon conversion of the Company's Series A
Convertible Preferred Stock, par value $.01 per share (the "Series A
Preferred Stock"), Series C Convertible Preferred Stock, par value $.01 per
share (the "Series C Preferred Stock"), and Series D Convertible Preferred
Stock, par value $.01 per share (the "Series D Preferred Stock"); and

                  WHEREAS, the Company and certain Investors (as defined
herein) have entered into a Series E Preferred Stock Purchase Agreement dated
March 27, 1998 whereby, upon the terms and subject to the conditions set
forth therein, such

<PAGE>



Investors have agreed to purchase an aggregate of 900,478 shares (the
"Preferred Shares") of the Company's Series E Convertible Preferred Stock,
par value $.01 per share (the "Series E Preferred Stock"); and

                  WHEREAS, the parties hereto, representing a majority of the
out standing shares of Common Stock and 66 2/3% of the outstanding shares of
Common Stock issued or issuable upon conversion of the Series A Preferred
Stock, Series C Preferred Stock and Series D Preferred Stock, desire to amend
and restate, as of the date hereof, the Amended Stockholders Agreement;

                  NOW, THEREFORE, in consideration of the foregoing and the
mutual covenants and agreements herein contained, and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the par ties hereto, intending to be legally bound, hereby
agree to amend and restate the Amended Stockholders Agreement to read in its
entirety as set forth herein:

                                    ARTICLE 1

                                   DEFINITIONS

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

                  Affiliate shall mean, with respect to any person, any other
person controlling, controlled by or under common control with such person.

                  Board shall mean the Board of Directors of the Company.

                  Canaan shall mean Canaan Ventures II Offshore C.V., a
Netherland Antilles limited partnership and Canaan Ventures II Limited
Partnership, a Delaware limited partnership.

                  Certificate of Incorporation shall mean the Amended and
Restated Certificate of Incorporation of the Company, as filed with the
Secretary of State of the State of Delaware.

                                        2

<PAGE>



                  Chase shall mean Chase Venture Capital Associates, L.P., a
California limited partnership, or one or more other funds affiliated with
Chase Capital Partners, a New York general partnership.

                  Commission shall mean the United States Securities and
Exchange Commission and any successor federal commission or agency having
similar powers.

                  Company Stock shall mean the Common Stock, the Preferred
Stock and any other common or preferred stock of the Company.

                  Effective Date shall mean the date of filing of the
Certificate of Incorporation with the Secretary of State of the State of
Delaware.

                  Equity Fund shall mean Whitney 1990 Equity Fund, L.P., a
Delaware limited partnership.

                  Exchange Act shall mean the Securities Exchange Act of
1934, as amended, and the rules and regulations thereunder.

                  Founders  shall mean Dr. Gefter, Dr. Signer and Dr.
Strosberg.

                  Greylock shall mean Greylock Limited Partnership, a
Delaware limited partnership.

                  Highland shall mean Highland Capital Partners II Limited
Partner ship, a Delaware limited partnership.

                  Independent Director shall mean a director of the Company
who is not (apart from such directorship) an officer, Affiliate, employee,
principal stockholder, consultant or partner of the Company, of any Affiliate
of the Company, of any Stockholder or of any entity that was dependent upon
the Company or any Stock holder for more than 5% of its revenues or earnings
in its most recent fiscal year.

                  Initial Public Offering shall mean when the Company shall
consummate a public offering of shares of Common Stock registered under the
Securities Act which yields aggregate gross proceeds to the Company of not
less than $20,000,000 and which is at a price per share of Common Stock equal
to or greater than the minimum price per share (in each case, the "Minimum
Per Share Offering Price") for each applicable time period set forth below.
The Minimum Per Share

                                        3

<PAGE>



Offering Price (which shall be subject to appropriate adjustment as set forth
in Section 4(f) of ARTICLE FIFTH of the Certificate of Incorporation) (i) for
the period commencing upon the Effective Date and ending at the close of
business on the first anniversary of the Effective Date shall be $16.80 per
share of Common Stock; (ii) for the period commencing on the day after the
first anniversary of the Effective Date and ending at the close of business
on the second anniversary of the Effective Date shall be $18.90 per share of
Common Stock; and (iii) at any time after the close of business on the second
anniversary of the Effective Date shall be $21.00 per share of Common Stock.
Notwithstanding the foregoing, in the event that the Company shall file with
the Securities and Exchange Commission a registration statement with respect
to a public offering of Common Stock on or before the close of business on
the second anniversary of the Effective Date, the Minimum Per Share Offering
Price applicable at the time of such filing of such registration statement
(the "Initial Filing Date") shall be the Minimum Per Share Offering Price for
the purposes of this definition of Initial Public Offering unless such public
offering is not consummated within 150 calendar days after the Initial
Filing Date.

                  Investors shall mean the Existing Investors, the New Investors
and their respective permitted transferees or assignees hereunder.

                  MIT shall mean Massachusetts Institute of Technology.

                  Person shall mean a corporation, association, partnership,
joint venture, organization, business, individual, trust or any other entity
or organization, including a government or any subdivision or agency thereof.

                  Preferred Stock shall mean the Series A Preferred Stock,
Series B Preferred Stock, Series C Preferred Stock, Series D Preferred Stock
and Series E Preferred Stock.

                  Registrable Securities shall mean (i) shares of Common
Stock issued or issuable, upon conversion of any Preferred Stock, (ii) the
shares of Common Stock owned by MIT and the Founders 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) they shall have been transferred pursuant to Rule 144, (iii)
they shall have been otherwise

                                        4

<PAGE>



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 the Company 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) they shall
have ceased to be outstanding.

                  Registration Expenses shall mean any and all out-of-pocket
expenses incident to the Company's performance of or compliance with Article
6 hereof, including, without limitation, all Commission, 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 Company Stock, all
printing expenses, the fees and disbursements of counsel for the Company 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 the holders of Registrable
Securities being registered to represent such group of holders (which counsel
shall be satisfactory to the holders of a majority of the shares of
Registrable Securities being registered), but excluding (a) any allocation of
the personnel or other general overhead expenses of the Company or other
expenses for the preparation of financial statements or other data normally
prepared by the Company in the ordinary course of its business, which shall
be borne by the Company in all cases, and (b) underwriting discounts and
commissions and applicable transfer and documentary stamp taxes, if any,
which shall be borne by the seller of the Registrable Securities in all cases.

                  Related Party shall mean:

                           (i) In the case of an individual, (A) such person,
         (B) such person's spouse, (C) the family members (i.e., spouse,
         parents, parents-in-law, issue, nephews, nieces, brothers,
         brothers-in-law, sisters, sisters-in-law, children-in-law and
         grandchildren-in-law) of such person and such person's spouse, (D) a
         trust for the benefit of any of the foregoing, (E) the estate or legal
         representatives of such person and (F) any corporation or partnership
         con trolled by such person;



                                        5

<PAGE>



                           (ii) In the case of a partnership, (A) such
         partnership and any of its partners (limited or general), (B) any
         retired partners of such partnership, (C) the estates or legal
         representatives of any such limited or general partners or retired
         partners, (D) any corporation or other business organization to which
         such partnership shall sell all or substantially all of its assets or
         with which it shall be merged and (E) any Affiliate of such
         partnership;

                           (iii) In the case of a corporation, (A) any such
         corporation and any of its subsidiaries, (B) any stockholder of such
         corporation, (C) any corporation or other business organization to
         which such corporation shall sell all or substantially all of its
         assets or with which it shall be merged and (D) any Affiliate of such
         corporation; and

                           (iv) In the case of the Series D Purchaser (as
         defined herein), in addition to the above, one or more of George Soros
         or Soros Fund Management LLC or affiliates thereof, and any person or
         entity for which any of George Soros, Soros Fund Management LLC or
         affiliates thereof act as investment advisor or investment manager.

                  Securities Act shall mean the Securities Act of 1933, as
amended, and the rules and regulations thereunder.

                  Series A Preferred Stock shall have the meaning specified
in the third preamble paragraph hereof.

                  Series B Preferred Stock shall mean the Company's Series B
Convertible Preferred Stock, par value $.01 per share.

                  Series C Preferred Stock shall have the meaning specified
in the third preamble paragraph hereof.

                  Series D Preferred Stock shall have the meaning specified
in the third preamble paragraph hereof.

                  Series D Purchaser shall have the meaning specified in
Section 5.3 hereof.

                                        6

<PAGE>



                  Series E Preferred Stock shall have the meaning specified
in the fourth preamble paragraph hereof.

                  Stockholder shall mean any of the Investors and the
Founders, as long as they hold any Common Stock, Preferred Stock or other
authorized and validly issued shares of any series of the Company's common or
preferred stock (including any permitted transferees or assignees hereunder
of any such holder).

                  Sutter Hill shall mean Sutter Hill Ventures, a California
limited partnership.

                  Underwritten Offering shall have the meaning specified in
Section 6.1.

                                    ARTICLE 2

                                    TRANSFERS

                  Section 2.1 Transfers. No Stockholder shall, directly or
indirectly, sell, assign, transfer, hypothecate, pledge, mortgage, encumber
or dispose of (each, a "Transfer") any shares of Company Stock except as
permitted by Section 2.2 hereof or Article 3 hereof.

                  Section 2.2 Permitted Transfers. (a) Any Stockholder may
Transfer any shares of Company Stock owned by such Stockholder to any of such
Stock holder's Related Parties.

                           (b) No Transfers of Company Stock shall be made
pursuant to the foregoing Section 2.2(a) unless, prior to the consummation of
any such Transfer, each such transferring Stockholder shall have caused the
transferee to execute and deliver to the Company a Stock Transfer Agreement
in the form attached hereto as Exhibit A.

                                        7

<PAGE>



                                    ARTICLE 3

                 COMPANY AND STOCKHOLDER RIGHT OF FIRST REFUSAL

                  Section 3.1 Right of First Refusal. If at any time any
Stockholder de sires to Transfer any shares of Company Stock to any Person
other than as permitted by Section 2.2 hereof and other than in connection
with a Sale of the Corporation (as defined in the Certificate of
Incorporation), such Stockholder (the "Offering Stockholder") will notify the
Company in writing (the "Notification") of such Stockholder's intention to do
so, specifying the number of shares of Company Stock proposed to be
transferred (the "Offered Shares"), the name of the Person or Persons to whom
such Stockholder proposes to Transfer the Offered Shares (or if no particular
purchaser is identified, then the general class of persons to whom such Stock
holder proposes to Transfer the Offered Shares), and a price per share which
shall be the minimum price at which such Stockholder proposes to effect the
Transfer (the "Minimum Price"). The Company shall promptly forward a copy of
the Notification to all other Stockholders. The Notification shall contain an
affirmation by the Offering Stockholder that such Stockholder has a
reasonable expectation of being able to effect the proposed Transfer to such
Person or Persons (or class of Persons) at the Minimum Price, and shall
recite the basis for such expectation. The Notification shall offer to sell
to the Company and, to the extent provided in this Article 3, the other
Stockholders, all of the Offered Shares, free and clear of any liens or
encumbrances in favor of third Persons, at the Minimum Price and on such
other terms and conditions, if any, not less favorable to the Company and the
other Stockholders than those proposed to be offered to such other Person or
Persons (or class of Persons). In the event all or any part of the
consideration for the proposed Transfer shall consist of other than cash, the
Minimum Price shall mean the fair value of such consideration, including the
fair value of any promissory notes of the prospective purchaser (taking into
account, among other matters, the terms of the notes and the credit
worthiness of the prospective purchaser).

                  Section 3.2 Notification by Company and Stockholders. The
Company and each Stockholder (other than the Offering Stockholder) shall have
the right and option (subject to the provisions of Section 3.3 hereof), for a
period of 30 days after delivery of the Notification, (i) in the case of the
Company, to elect to purchase all or any part of the Offered Shares at the
Minimum Price and on the terms and conditions stated therein and (ii) in the
case of each such Stockholder, to elect to purchase at the Minimum Price and
on the terms and conditions stated therein (A) up to its Proportionate
Percentage (as defined below) of any Offered Shares which the

                                        8

<PAGE>



Company does not elect to purchase ("Remaining Shares") and (B) if such Stock
holder elects to purchase its full Proportionate Percentage, all or any part
of any other Stockholders' Proportionate Percentages of any Remaining Shares
to the extent that such other Stockholders do not elect to purchase their
full Proportionate Percentages of the Remaining Shares. "Proportionate
Percentage" shall mean as to each Stock holder, the percentage figure which
expresses the ratio between the number of shares of outstanding Company Stock
(on an as-converted basis) owned by such Stock holder and the aggregate
number of such shares of Company Stock (on an as-converted basis) owned by
all Stockholders with respect to which the calculation is being made. For
purposes of computing a Stockholder's Proportionate Percentage, each
Stockholder which is a partnership shall be deemed to be the owner of all
shares of Company Stock which have been transferred to and are held by such
Stockholder's Related Parties and the Related Parties of such Related
Parties. Such elections by the Company and such other Stockholders shall be
made by delivering a written notice to the Offering Stockholder within the
aforesaid 30-day period. The Company shall be entitled to purchase all
Offered Shares which it elects to purchase. Each such Stockholder shall be
entitled to purchase up to (i) its Proportionate Percentage of any Remaining
Shares and (ii) the Remaining Shares which such Stockholder elects to
purchase pursuant to clause (ii)(B) of the first sentence of this Section
3.2. If the Stockholders who elect to purchase their full Proportionate
Percentages of any Remaining Shares also elect to purchase in the aggregate
more than 100% of any Remaining Shares referred to in clause (ii)(B) of the
first sentence of this Section 3.2, such Remaining Shares will be sold to
such Stockholders in accordance with their respective Proportionate
Percentages; provided, however, that no such Stockholder may purchase a
greater number of such Remaining Shares than the amount which such
Stockholder elects to purchase pursuant to clause (ii)(B) of the first
sentence of this Section 3.2. Sales of Offered Shares under the terms of this
Section 3.2 hereof shall be made at the offices of the Company on a mutually
satisfactory business day within 30 days after the earlier of (i) the
expiration of the 30-day period referred to in Section 3.2 hereof, or (ii)
the receipt of written notices from the Company and all other Stockholders
responding to the Notification. Delivery of certificates or other instruments
evidencing such Offered Shares duly endorsed for transfer to the Company or
the other Stockholders shall be made on such date against payment of the
purchase price therefor.

                  Section 3.3 Third Party Sales. If effective acceptance
shall not be received pursuant to Section 3.2 hereof with respect to all
Offered Shares pursuant to any Notification, then the Offering Stockholder
may sell all or any part of such Offered Shares as to which such an effective
acceptance has not been so received

                                        9

<PAGE>



(the "Saleable Offered Shares") at a price not less than the Minimum Price,
and on other terms not materially more favorable to the purchaser thereof
than the terms stated in such Notification, at any time within 90 days after
the expiration of the offer pursuant to the Notification. In the event such
Saleable Offered Shares are not sold by the Offering Stockholder during such
90-day period, the right of the Offering Stockholder to sell such Saleable
Offered Shares shall expire and the obligations of this Article 3 shall be
reinstated; provided, however, that in the event the Offering Stockholder
determines, at any time during such 90-day period, that the sale of all or
any part of the Saleable Offered Shares on the terms set forth in the
preceding sentence is impractical, the Offering Stockholder can terminate the
offer and reinstate the procedure provided in this Article 3, without waiting
for the expiration of such 90-day period. The Offering Stockholder may
specify in the Notification that all Offered Shares must be sold, in which
case acceptances received pursuant to Section 3.2 hereof shall be deemed
conditioned upon (i) receipt of written notices of acceptance with respect
to all Offered Shares mentioned in such Notification and/or (ii) the sale of
all or the remaining Offered Shares pursuant to this Section 3.3.

                  Section 3.4 Stock Transfer Agreement. It shall be a
condition of any Transfer of Offered Shares to a third Person or Persons that
the Offering Stockholder shall cause such Person or Persons to execute and
deliver to the Company a Stock Transfer Agreement in the form attached hereto
as Exhibit A.

                  Section 3.5 Legend. Each certificate representing shares of
Company Stock shall be stamped or otherwise imprinted with a legend
substantially in the following form:

         "The securities 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 said Act or an exemption from registration is available. The
         securities represented by this certificate are subject to and shall be
         transferable only upon the terms and conditions of an Amended and
         Restated Stockholders Agreement by and among the issuer and certain
         stock holders of the issuer, dated as of April 30, 1998, a copy of
         which is on file with the Secretary of the issuer."

                  Section 3.6 Restrictions on Transferability.
Notwithstanding any other provision of this Agreement, Company Stock shall
not be Transferred, and the Company shall not be required to register any
Transfer of Company Stock on the

                                       10

<PAGE>



books of the Company, unless the Company shall have been provided with an
opinion of counsel reasonably satisfactory to it prior to such Transfer that
registration under the Securities Act is not required in connection with the
transaction resulting in such Transfer; provided, however, that no such
opinion of counsel shall be required for a Transfer by an Investor which is a
partnership to a partner of such Purchaser or a retired partner of such
Purchaser who retires after the date hereof, or to the estate of any such
partner or retired partner, or a Transfer by gift, will or intestate
succession from any such partner to his spouse or members of his or his
spouse's family, if the transferee executes and delivers to the Company a
Stock Transfer Agreement in the form attached hereto as Exhibit A except if
the Company reasonably determines that it is not free from doubt as to
whether an exemption from registration under the Securities Act is available
for any such proposed Transfer.

                  Section 3.7 No Recognition of Transfer. The Company agrees
that it will not at any time permit any Transfer to be made on its books or
records of the certificates representing shares of Company Stock subject to
this Agreement, and that it will refuse to recognize any claims of ownership
of any purported direct or indirect transferee of such Company Stock, unless
such Transfer is made pursuant to and in accordance with the terms and
conditions of this Agreement.

                                    ARTICLE 4

                               BOARD OF DIRECTORS

                  Section 4.1 Size of Board; Composition; Observer Rights.
The current members of the Board are: Dr. Gefter, Henry F. McCance, William
Laverack, Jr., David Sharrock, Albert L. Zesiger, G. Leonard Baker and Damion
E. Wicker. Each of Highland and Canaan (i) shall be entitled to have a
representative attend all meetings of the Board, and (ii) will be sent copies
of all notices of Board meetings and actions.

                  Section 4.2 Voting Agreement. (a) At every annual or
special meeting called (or written consent solicited) for the purpose of
electing directors of the Company, the Company shall use its best efforts to
cause to be nominated for election and elected as a director of the Company,
and thereafter to continue in office, (i) Dr. Gefter (so long as he is an
employee of or consultant to the Company), (ii) one person designated by
Equity Fund (the "Whitney Designee"), (iii) one person designated by
Greylock (the "Greylock Designee"), (iv) one person designated by Sutter

                                       11

<PAGE>



Hill (the "Sutter Hill Designee") and (v) one person designated by Chase (the
"Chase Designee"). The current Whitney Designee is William Laverack, Jr., the
current Greylock Designee is Henry F. McCance, the current Sutter Hill
Designee is G. Leonard Baker, Jr. and the current Chase designee is Damion E.
Wicker. The Whitney Designee, the Greylock Designee, the Sutter Hill
Designee and the Chase Designee together are referred to collectively herein
as the "Designees" and individually as a "Designee". The current Independent
Director is David Sharrock.

                           (b) Each Stockholder entitled to vote agrees to
vote (or cause to be voted) all shares of Company Stock then owned by such
Stockholder so as to continue in office Dr. Gefter (so long as he is an
employee of, or consultant to, the Company) and the Designees.

                           (c) Each of Greylock, Equity Fund, Sutter Hill and
Chase may at any time by written notice to the Company designate their
respective Designees for removal as a member of the Board with or without
cause and, upon such designation, (i) each Stockholder entitled to vote
agrees to vote all shares of Company Stock then owned by such Stockholder to
effect such removal and (ii) the Stock holders and the Company shall use
their respective best efforts to effect, or cause the stockholders of the
Company to effect, such removal.

                           (d) If there is a vacancy in the Board caused by
the death, resignation or removal of a Designee, (i) each Stockholder
entitled to vote agrees to vote all shares of Company Stock then owned by
such Stockholder to elect to the Board, and thereafter to vote all such
shares to continue in office, such substitute director as the Person or
entity entitled to designate such Designee may designate and (ii) the Company
shall use its best efforts to elect or nominate for election by stockholders,
and thereafter to continue in office, such substitute director as the Person
or entity entitled to designate such Designee may so designate.

                                   ARTICLE 5

                              RIGHT OF FIRST OFFER

                  Section 5.1 Right of First Offer. The Company grants to
each of the Stockholders the right of first offer to purchase (a) up to such
Stockholder's pro-rata share, as defined below, of any equity securities of
the Company, including shares of Common Stock or securities of any type
convertible into, or entitling the holder

                                       12

<PAGE>



thereof to purchase shares of, Common Stock, proposed to be issued by the
Company subsequent to the date hereof (such securities being sometimes
hereinafter referred to as the "Equity Securities"), and (b) to the extent
specified in Section 5.2 hereof, all or any part of any other Stockholders'
respective pro-rata shares of such Equity Securities to the extent that such
other Stockholders do not elect to purchase their full pro-rata shares. A
Stockholder's "pro-rata share" shall be that portion of the Equity Securities
proposed to be issued which bears the same relation to all of the Equity
Securities proposed to be issued as the number of shares of Common Stock held
by such Stockholder bears to the outstanding shares of Common Stock held by
all Stockholders (assuming in each case the conversion of all outstanding
securities which are convertible into Common Stock, all determined
immediately prior to the offering of the Equity Securities). For purposes of
computing an Investor's pro-rata share, each Investor which is a partnership
shall be deemed to be the owner of all shares of Company Stock which have
been transferred to and are held by such Investor's Related Parties and the
Related Parties of such Related Parties in accordance with Section 2.2.

                  Section 5.2 Notice. In the event that the Company proposes
to undertake an issue of Equity Securities, it shall deliver to each
Stockholder written notice of its intention, describing such Equity
Securities, specifying such Stock holder's pro-rata share and stating the
purchase price and other material terms upon which it proposes to issue the
same (the "First Offer Notice"). For a period of 20 days from the date the
First Offer Notice is given, each Stockholder shall have the right to elect,
by written notice to the Company, to purchase (a) all or any portion of its
pro-rata share of the Equity Securities described in the First Offer Notice
and (b) if such Stockholder elects to purchase its full pro-rata share, all
or any part of the pro-rata share of any other Stockholder to the extent that
such other Stockholder does not elect to purchase its full pro-rata share,
upon the terms and condition specified in the First Offer Notice. If the
Stockholders who elect to purchase their full pro-rata shares also elect to
purchase in the aggregate more than 100% of the Equity Securities referred
to in clause (b) of the preceding sentence, the Equity Securities referred to
in clause (b) of the preceding sentence will be sold to such Stockholders in
accordance with their respective pro-rata shares; provided, however, that no
Stock holder may purchase a greater amount of such Equity Securities than the
amount which such Stockholder elects to purchase pursuant to clause (b) of
the preceding sentence. In the event the Stockholders fail to exercise their
right of first offer within the specified period, or the Stockholders elect
to acquire less than their aggregate pro-rata shares pursuant to the exercise
of such right, then, during the 90-day period following the expiration of
such 20-day period, the Company may sell, free of any

                                       13

<PAGE>



right of first offer on the Stockholders' part, the portion of the
Stockholders' pro-rata shares not purchased pursuant to such right of first
offer (the "Third Party Shares"), at not less than the price, and on other
terms not more favorable to the purchasers in any material respect than the
other terms, stated in the First Offer Notice. The closing of sales of
Equity Securities to the Stockholders pursuant to this Article 5 shall take
place at the offices of the Company on a mutually satisfactory business day
within 15 days after the expiration of the aforesaid 20-day period, or, at
the Company's election, at the same time as the closing of the issuance and
sale of any Third Party Shares.

                  Section 5.3 Exceptions. The right of first offer granted
under this Article V shall not apply to (i) any shares of Common Stock issued
upon conversion of the Preferred Stock; (ii) the issuance of Equity
Securities to employees, consultants, directors or persons performing
similar functions, if such issuances are approved by the Board or such
issuances are pursuant to a plan or plans approved by the Board; (iii) the
issuance of Equity Securities upon a stock split, stock dividend or
combination with respect to the Common Stock; (iv) the issuance of any
Purchase Rights (as defined in the Certificate of Incorporation) pro-rata to
recordholders of Common Stock and Preferred Stock, or the issuance of Equity
Securities upon the conversion, exercise or exchange of any Purchase Rights;
(v) the issuance of Equity Securities upon the conversion, exercise or
exchange of any outstanding Equity Securities to which the right of first
offer set forth in this Article 5 applied at the time of issuance thereof;
(vi) the issuance of any Equity Securities in a public offering pursuant to a
registration statement under the Securities Act; (vii) the issuance of Equity
Securities pursuant to the acquisition by the Company of another corporation
by merger, acquisition of more than 51% of such corporation's outstanding
stock or purchase of all or substantially all of its assets; (viii) the
issuance of Equity Securities in connection with a strategic business
arrangement or lease arrangement in which the Person receiving such Equity
Securities or an Affiliate of such Person enters into a contractual
relationship with the Company relating to research and development,
manufacturing, marketing, licensing or leasing, which issuance and
transaction are approved in good faith by the Board; (ix) the issuance of
Series D Convertible Preferred Stock, par value $.01 per share, of the
Company pursuant to a Series D Preferred Stock Purchase Agreement dated June
9, 1997 by and between the Company and the purchaser listed on Schedule I
thereto (the "Series D Purchaser"); or (x) the issuance of Series E
Convertible Preferred Stock, par value $.01 per share, of the Company
pursuant to a Series E Preferred Stock Purchase Agreement dated March 27,
1998 by and among the Company and the purchasers listed on Schedules A and B
thereto.

                                       14

<PAGE>




                                   ARTICLE 6

                               REGISTRATION RIGHTS

                  Section 6.1 Demand Registration. (a) After the first to
occur of the consummation of the Initial Public Offering and December 23,
1996, upon the writ ten request of any holder or holders of at least 662/3%
of the outstanding Registrable Securities issued or issuable upon conversion
of the Series A Preferred Stock, Series C Preferred Stock, Series D Preferred
Stock and Series E Preferred Stock requesting that the Company effect the
registration under the Securities Act of all or part of such holder's or
holders' Registrable Securities and specifying the number thereof to be
registered by each such holder and the intended method of disposition
thereof, the Company will, as expeditiously as possible, give written notice
of such request to all holders of Registrable Securities, and shall thereupon
use its reasonable best efforts to effect the registration under the
Securities Act, subject to Section 6.1(e), of:

                  (i) the Registrable Securities which the Com
         pany has been so requested to register by such holder or holders; and

                 (ii) all other Registrable Securities which the
         Company has been requested to register by any other holder thereof by
         written request given to the Company within 30 calendar days after the
         giving of such written notice by the Company (which request shall
         specify the intended method of disposition of such Registrable
         Securities),

all to the extent necessary to permit the disposition (in accordance with the
intended methods thereof as aforesaid) of the Registrable Securities so to be
registered. Notwithstanding the foregoing provisions of subsection 6.1(a),
the Company shall not be obligated to file more than three registration
statements pursuant to this Section 6.1(a) and shall not be obligated to
file any registration statement pursuant to this Section 6.1(a) where the
proposed aggregate offering price of the securities to be sold thereunder is
less than $5 million.

                           (b) After the first anniversary of the Initial
Public Offering, upon the written request of any holder or holders of a
majority of the outstanding Registrable Securities issued or issuable upon
conversion of the Series E Preferred Stock (the "Requesting Series E
Holders") requesting that the Company effect the

                                       15

<PAGE>



registration under the Securities Act of all or part of the Requesting Series
E Holders' Registrable Securities and specifying the number thereof to be
registered by each such holder and the intended method of disposition
thereof, the Company will, as expeditiously as possible, give written notice
of such request to all holders of Registrable Securities, and shall
thereupon use its reasonable best efforts to effect the registration under
the Securities Act, subject to Section 6.1(f), of:

                  (i) the Registrable Securities which the Com
         pany has been so requested to register by the Requesting Series E
         Holders; and

                 (ii) all other Registrable Securities which the
         Company has been requested to register by any other holder thereof by
         written request given to the Company within 30 calendar days after the
         giving of such written notice by the Company (which request shall
         specify the intended method of disposition of such Registrable
         Securities),

all to the extent necessary to permit the disposition (in accordance with the
intended methods thereof as aforesaid) of the Registrable Securities so to be
registered. Notwithstanding the foregoing provisions of subsection 6.1(b),
the Company shall not be obligated to file more than one registration
statement pursuant to this Section 6.1(b) and shall not be obligated to file
any registration statement pursuant to this Section 6.1(b) where the proposed
aggregate offering price of the securities to be sold thereunder is less than
$5 million.

                           (c) At such time as the Company shall have
qualified for the use of Form S-3 (or any similar form or forms promulgated
by the Commission), the holders of 10% of the then outstanding Registrable
Securities shall have the right to request the registration of Registrable
Securities on Form S-3. The Company shall give prompt written notice of each
such proposed registration to all other record holders of Registrable
Securities. Subject to Section 6.1(e) hereof, such other holders shall have
the right, by giving written notice to the Company within 30 days from
receipt of the Company's notice, to elect to have included in such
registration such of their Registrable Securities as such holders may request
in such notice of election. Thereupon, the Company shall, as expeditiously as
possible, use its reasonable best efforts to effect the registration on Form
S-3 of all Registrable Securities which the Company has been requested to
register; provided, however, that the Company shall not be obligated to file
and use its reasonable best efforts to

                                       16

<PAGE>



cause to become effective (i) more than one registration on Form S-3 in any
one year period or (ii) any such registration statement where the proposed
aggregate offering price of the securities to be sold thereunder is less than
$2 million. In addition, the Company shall not be obligated to file and use
its reasonable best efforts to cause to become effective more than three
registration statements pursuant to which Registrable Securities are to be
sold pursuant to this Section 6.1(b). Three registrations effected on Form
S-3 pursuant to this Section 6.1(b) shall not be counted as demand
registrations pursuant to Section 6.1(a) hereof.

                           (d) At such time as the Company shall have
qualified for the use of Form S-3 (or any similar form or forms promulgated
by the Commission), the Requesting Series E Holders shall have the right to
request the registration of Registrable Securities on Form S-3. The Company
shall give prompt written notice of such proposed registration to all other
record holders of Registrable Securities. Subject to Section 6.1(f) hereof,
such other holders shall have the right, by giving written notice to the
Company within 30 days from receipt of the Company's notice, to elect to have
included in such registration such of their Registrable Securities as such
holders may request in such notice of election. Thereupon, the Company shall,
as expeditiously as possible, use its reasonable best efforts to effect the
registration on Form S-3 of all Registrable Securities which the Company has
been requested to register; provided, however, that the Company shall not be
obligated to file and use its reasonable best efforts to cause to become
effective (i) more than one registration on Form S-3 pursuant to this Section
6.1(d) or (ii) any such registration statement where the proposed aggregate
offering price of the securities to be sold thereunder is less than $2
million. A registration effected pursuant to this Section 6.1(d) shall not be
counted as a demand registration pursuant to Section 6.1(b) hereof.

                           (e) The Company may include in a registration
requested under Sections 6.1(a) or 6.1(c) any authorized but unissued shares
of Common Stock for sale by the Company; provided, however, that such shares
shall not be included to the extent that the holders of a majority of the
Registrable Securities included therein determine in good faith that the
inclusion of such shares will interfere with the successful marketing of the
Registrable Securities to be included therein. If the offering to which a
registration statement under either Section 6.1(a) or Section 6.1(c) relates
is an underwritten offering (an "Underwritten Offering"), and if, after all
shares of Common Stock proposed to be offered by the Company have been
excluded from such registration, a greater number of Registrable Securities
is offered for participation in such underwriting than in the opinion of the
investment banker or bankers that shall manage the offering (the "Managing
Underwriter") can be accom-

                                       17
<PAGE>

modated without adversely affecting the underwriting, the amount of
Registrable Securities proposed to be offered in the underwriting shall be
reduced, pro-rata (based upon the amount of Registrable Securities owned)
among the holders requesting the inclusion of Registrable Securities in such
registration, to a number deemed satisfactory by the Managing Underwriter;
provided, however, that for purposes of making any such reduction, each
Investor which is a partnership, together with the affiliates, partners and
retired partners of such Investor, the estates and family members of any
such partners and retired partners and of their spouses, and any trusts for
the benefit of any of the foregoing Persons shall be deemed to be a single
"holder" of Registrable Securities and any pro-rata reduction with respect to
such "holder" shall be based upon the aggregate amount of Registrable
Securities owned by all entities and individuals included in such "holder",
as defined in this proviso (and the aggregate amount so allocated to such
"holder" shall be allocated among the entities and individuals included in
such "holder" in such manner as such Investor may reason ably determine); and
provided, further, that any such reduction may be greater than pro-rata with
respect to Registrable Securities held by officers, employees and consultants
of the Company if, in the reasonable opinion of the Managing Under writer,
without such greater reduction, the proposed offering could not proceed on
terms reasonably acceptable to the Managing Underwriter.

                           (f) The Company may include in a registration
requested under Sections 6.1(b) or 6.1(d) any authorized but unissued shares
of Common Stock for sale by the Company; provided, however, that such shares
shall not be included to the extent that the holders of a majority of the
Registrable Securities issued or issuable upon conversion of the Series E
Preferred Stock included therein determine in good faith that the inclusion
of such shares will interfere with the successful marketing of the
Registrable Securities to be included therein. If the offering to which a
registration statement under either Section 6.1(b) or Section 6.1(d) relates
is an Underwritten Offering, and if, after all shares of Common Stock
proposed to be offered by the Company have been excluded from such
registration, a greater number of Registrable Securities is offered for
participation in such underwriting than in the opinion of the Requesting
Series E Holders, based on advice of the Managing Underwriter, can be
accommodated without adversely affecting the underwriting, the amount of
Registrable Securities proposed to be offered in the underwriting shall be
reduced by excluding on a pro-rata basis (based upon the amount of
Registrable Securities owned, provided, however, that any such reduction may
be greater than pro-rata with respect to Registrable Securities held by
officers, employees and consultants of the Company if, in the reasonable
opinion of the Requesting Series E Holders, based on advice of the Managing
Underwriter, without such greater

                                       18
<PAGE>



reduction, the proposed offering could not proceed on terms reasonably
acceptable to the Requesting Series E Holders) Registrable Securities offered
to be included in such registration which are held by persons (the
"Non-Series E Holders") other than the holders of Registrable Securities
issued or issuable upon conversion of the Series E Preferred Stock to a
number which is reasonably satisfactory to the Requesting Series E Holders,
based on advice of the Managing Underwriter. If after all Registrable
Securities held by Non-Series E Holders have been excluded from such
registration, a greater number of Registrable Securities is offered for
participation in such underwriting than in the opinion of the Managing
Underwriter can be accom modated without adversely affecting the
underwriting, the amount of Registrable Securities proposed to be offered in
the underwriting shall be reduced, pro-rata (based upon the amount of
Registrable Securities owned) among the holders of Registrable Securities
issued or issuable upon conversion of the Series E Preferred Stock requesting
the inclusion of Registrable Securities in such registration, to a number
deemed satisfactory by the Managing Underwriter; provided, however, that for
purposes of making any such reduction referred to in this Section 6.1(f),
each Investor which is a partnership, together with the affiliates, partners
and retired partners of such Investor, the estates and family members of any
such partners and retired partners and of their spouses, and any trusts for
the benefit of any of the foregoing Persons shall be deemed to be a single
"holder" of Registrable Securities and any pro-rata reduction with respect to
such "holder" shall be based upon the aggregate amount of Registrable
Securities owned by all entities and individuals included in such "holder",
as defined in this proviso (and the aggregate amount so allocated to such
"holder" shall be allocated among the entities and individuals included in
such "holder" in such manner as such Investor may reasonably deter mine).

                           (g) A registration requested pursuant to this
Section 6.1 will not be deemed to have been effected unless it has become
effective; provided that if, after it has become effective, the offering of
Registrable Securities pursuant to such registration is interfered with by
any stop order, injunction or other order or requirement of the Commission
or other governmental agency or court which prevents the successful
completion of such offering, such registration will be deemed not to have
been effected.

                           (h) The Company will pay all Registration Expenses
in connection with any registration of Registrable Securities effected by it
pursuant to this Section 6.1.

                                       19

<PAGE>



                           (i) If a requested registration pursuant to this
Section 6.1 involves an Underwritten Offering, the Persons holding a majority
of the Registrable Securities to be included in such registration shall have
the right to select the Managing Underwriter, subject to the approval of the
Company which shall not be unreasonably withheld.

                           (j) If at the time a registration request pursuant
to this Section 6.1 is made or is pending, the Board determines that the
filing of a registration statement would require the disclosure of material
information regarding a possible financing, business combination or other
material transaction, which disclosure the Board determines in its good faith
judgment would be detrimental to the Company, the Company may delay a
registration pursuant to this Section 6.1 until the date upon which such
material information is disclosed to the public or ceases to be material.

                           (k) Notwithstanding the foregoing, in the event
that the Company intends to commence a public offering of securities to which
Section 6.2 hereof will apply, it shall so notify the holders of Registrable
Securities in writing and such holders shall be deemed to have waived their
rights to demand registration under this Section 6.1 for a period of 120 days
following such notice.

                  Section 6.2 Piggyback Registration. (a) If at any time the
Company proposes to register (other than a registration pursuant to Section
6.1 hereof) any of its Equity Securities 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 could be registered for sale by the
holders thereof (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 the Company 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), the Company shall on each such occasion
give written notice to all holders of Registrable Securities of its intention
so to do, describing such Equity Securities and specifying the form and
manner and the other relevant facts involved in such proposed registration
(including, without limitation, (x) whether or not such pro posed registered
offering will be an underwritten offering and, if so, the identity of 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

                                       20
<PAGE>

Underwriter. Upon the written request of any such holder delivered to the
Company within 30 calendar days after the receipt of any such notice (which
request shall specify the Registrable Securities intended to be disposed of by
such holder and the intended method of disposition thereof), the Company will
use its reasonable best efforts to effect the registration under the Securities
Act of all of the Registrable Securities that the Company 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 in connection with
         such registration, the Company shall determine for any reason not to
         register such securities, the Company may, at its election, give
         written notice of such determination to each holder of Registrable
         Securities who made a request as hereinabove provided 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 in connection therewith),
         without prejudice, however, to the rights, if any, of any one or more
         holders to request that such registration be effected as a registration
         under Section 6.1; and

                 (ii) If such registration involves an Underwritten Offering,
         all holders of Registrable Securities requesting to be included in the
         Company's registration must sell their Registrable Securities to the
         underwriters selected by the Company on the same terms and conditions
         as apply to the Company.

                  No registration effected under this Section 6.2 shall relieve
the Company of its obligation to effect any registration upon request under
Section 6.1.

                           (b) The Registration Expenses incurred in connection
with each registration of Registrable Securities requested pursuant to this
Section 6.2 shall be paid by the Company.

                           (c) If a registration pursuant to this Section 6.2
involves an Underwritten Offering and the Managing Underwriter advises the
Company that, in its opinion, the number of shares proposed to be included in
such registration should be limited due to market conditions, then the Company
will include in such registration to the extent of the number which the Company
is so advised can be sold in such


                                       21
<PAGE>

offering (i) first, the securities the Company proposes to sell and (ii) second,
the number of shares of Common Stock 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 is
offered for inclusion in the proposed underwriting than in the opinion of the
Managing Underwriter proposing to underwrite securities to be sold by the
Company can be accommodated without adversely affecting the proposed
underwriting, the Company may elect to reduce pro-rata (based upon the amount of
shares owned which carry registration rights) 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 the Company to a number
deemed satisfactory by the Managing Underwriter. With respect to the proviso of
the preceding sentence, if the Company elects to reduce pro-rata the amount of
securities proposed to be offered in the underwriting for the accounts of all
Persons other than the Company, for purposes of making any such reduction, (i)
each Investor which is a partnership, together with the Affiliates, partners and
retired partners of such Investor, the estates and family members of any such
partners and retired partners and of their spouses, and any trusts for the
benefit of any of the foregoing Persons shall be deemed to be a single "Person",
and any pro-rata reduction with respect to such "Person" shall be based upon
the aggregate amount of shares carrying registration rights owned by all
entities and individuals included as such "Person", as defined in this sentence
(and the aggregate amount so allocated to such "Person" shall be allocated among
the entities and individuals included in such "Person" in such manner as such
Investor may reasonably determine), and (ii) any such reduction may be greater
than pro-rata with respect to Registrable Securities held by officers, employees
and consultants of the Company if, in the reasonable opinion of the Managing
Underwriter without such greater reduction, the proposed offering could not
proceed on terms reasonable acceptable to the Managing Underwriter.

                           (d) In connection with any Underwritten Offering with
respect to which holders of Registrable Securities shall have requested
registration pursuant to subsection 6.2(a), the Company shall have the right to
select the Managing Underwriter with respect to the offering.

                  Section 6.3 Registration Procedures. (a) If and whenever the
Company is required to use its reasonable best efforts to effect or cause the
registration of any Registrable Securities under the Securities Act as provided
in Section 6.1 or 6.2, the Company will, as expeditiously as possible:



                                       22
<PAGE>

                  (i) Prepare and, in any event within 60 calendar days after
         the end of the period within which requests for registration may be
         given to the Company, file with the Commission a registration
         statement with respect to such Registrable Securities and use its
         reasonable best efforts to cause such registration statement to become
         and remain effective; provided, however, that the Company may
         discontinue any registration of its securities that is being effected
         pursuant to Section 6.2 at any time prior to the effective date of the
         registration statement relating thereto.

                  (ii) Prepare and file with the Commission 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 ninety 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 the seller or sellers
         thereof set forth in such registration statement.

                  (iii) Furnish to each holder of Registrable Securities covered
         by the registration statement 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.

                  (iv) 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 the seller or sellers thereof to
         consummate the disposition of such Registrable Securities.

                  (v) Immediately notify each seller of Registrable Securities
         covered by such registration statement, at any time when a prospectus
         relating thereto is required to be delivered under the Securities Act
         within the appropriate period mentioned in


                                       23

<PAGE>

         Section 6.3(a)(ii), if the Company 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 any such seller, 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.

                  (vi) Otherwise use its best efforts to comply with all
         applicable rules and regulations of the Commission and make generally
         available to its security holders, in each case as soon as practicable,
         but not later than 45 calendar days after the close of the period
         covered thereby (90 calendar days in case the period covered
         corresponds to a fiscal year of the Company), an earnings statement of
         the Company which will satisfy the provisions of subsection 11(a) of
         the Securities Act.

                  (vii) 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 the Company in customary form and
         covering such matters as are customarily covered by such letters.

                  (viii) 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) Each holder of Registrable Securities will, upon
receipt of any notice from the Company of the happening of any event of the kind
described in subsection 6.3(a)(v), forthwith discontinue disposition of the
Registrable Securities pursuant to the registration statement covering such
Registrable Securities until such


                                       24

<PAGE>

holder's receipt of the copies of the supplemented or amended prospectus
contemplated by Section 6.3(a)(v).

                           (c) If a registration pursuant to Section 6.1 or 6.2
involves an Underwritten Offering, each holder of Registrable Securities,
whether or not such holder'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 and the Managing Underwriter shall
reasonably determine is required to effect a successful offering.

                           (d) If a registration pursuant to Section 6.1 or 6.2
involves an Underwritten Offering, the Company 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 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.

                  Section 6.4 Indemnification. (a) In the event of any
registration of any securities of the Company under the Securities Act, the
Company will, and hereby does, indemnify and hold harmless the holder of 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 such
holder or any such underwriter within the meaning of the Securities Act against
any losses, claims, damages or liabilities, joint or several, to which such
holder 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
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

                                       25

<PAGE>

any document filed in connection therewith or in connection with any
qualification pursuant to Section 6.3(a)(iv), 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 the Company
will reimburse such holder 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 the Company shall not be liable
with respect to any holder of Registrable Securities 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 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 incident to registration or qualification of
any Registrable Securities pursuant to Section 6.3(a)(iv), in reliance upon and
in conformity with written information furnished to the Company by the holder
of Registrable Securities specifically for use in the preparation thereof; and
provided, further, that the Company 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 such holder of
Registrable Securities or any such director, officer, underwriter or controlling
person and shall survive the transfer of such securities by such holder.

                           (b)The Company may require, as a condition to
including any Registrable Securities in any registration statement filed
pursuant to Section 6.3, that the Company shall have received an undertaking
satisfactory to it from the prospective seller of such Registrable Securities,
to indemnify and hold harmless (in the same manner and to the same extent as set
forth in subdivision (a) of this Section 6.4) the Company, each director of the
Company, each officer of the Company and each other Person, if any, who controls
the Company 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 con-


                                       26
<PAGE>

tained therein, or any amendment or supplement thereto, or any qualification
pursuant to Section 6.3(a)(iv), if such statement or omission was made in
reliance upon and in conformity with written information furnished to the
Company by the seller of such Registrable Securities 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 the Company or any such director, officer or controlling person and
shall survive the Transfer of such securities by such seller. In no event shall
the liability of any such seller of Registrable Securities hereunder be greater
in amount than the dollar amount of the proceeds received by such seller 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 subdivisions of this Section 6.4, 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 subdivisions of this Section 6.4, 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
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)

                                       27

<PAGE>

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 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.

                  Section 6.5 Contribution. In order to provide for just and
equitable contribution in circumstances under which the indemnity contemplated
by Section 6.4 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 the Company, any seller of Registrable Securities 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 Registrable Securities (taking into account the portion of 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. The Company and
each seller of Registrable Securities agree with each other that no seller of
Registrable Securities shall be required to contribute any amount in excess of
the amount such seller would have been required to pay to an indemnified party
if the indemnity under subsection 6.4(b) was available. The Company and each
such seller agree with each other and the underwriters of the Registrable
Securities, if requested 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 6.5, 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 the Company who signed the registration statement,
and each Person, if any, who controls the Company or a seller of Registrable
Securities within the meaning of Section 15 of the Securities Act shall have
the same rights


                                       28

<PAGE>

to contribution as the Company or such seller of Registrable Securities, as the
case may be.

                  Section 6.6 Rule 144. If the Company 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, the Company 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 Commission thereunder (or, if the Company is not
required to file such reports, it will, upon the request of any holder of
Registrable Securities, make publicly available other information), and it will
take such further action as any holder of Registrable Securities may reasonably
request, all to the extent required from time to time to enable such holder to
sell shares of Registrable Securities without registration under the Securities
Act within the limitation of the exemptions provided by (i) Rule 144 under the
Securities Act, as such Rule may be amended from time to time, or (ii) any
similar rule or regulation hereafter adopted by the Commission. Upon the request
of any holder of Registrable Securities, the Company will deliver to such holder
a written statement as to whether it has complied with such requirements.

                                    ARTICLE 7

                                FUTURE INVESTORS

                  If subsequent to the date hereof, the Company grants to
holders or prospective holders of its securities registration rights which are
more favorable than the terms or provisions of Article 6 of this Agreement are
to the Stockholders, such Article 6 shall be deemed to be automatically amended
(without the necessity of any action on the part of the Company or the
Stockholders) to grant to the Stockholders such more favorable registration
rights, in addition to those set forth herein. If subsequent to the date hereof,
the Company grants to holders or prospective holders of its securities
antidilution rights which are more favorable than the terms or provisions of the
Certificate of Incorporation are to the holders of Preferred Stock, this
Agreement shall be deemed to be automatically amended (without the necessity of
any action on the part of the Company or any holder of Preferred Stock) to grant
to the holders of Preferred Stock such more favorable antidilution rights, in
addition to those set forth in the Certificate of Incorporation, it being
understood and agreed that antidilution rights which provide that the conversion
price of a class or series of Equity Securities may not be adjusted such that it
is less than the applicable conversion price of another class or series of
Equity Securities shall not be deemed to be


                                       29

<PAGE>

more favorable than the antidilution rights of such other class or series of
Equity Securities. In connection therewith, the Company shall take all action
available to it to cause the Certificate of Incorporation to be amended to
provide for such more favorable antidilution rights, and each Stockholder agrees
to vote (or cause to be voted) all shares of Company Stock then owned by such
Stockholder to approve such amendment. Such amended antidilution provisions
shall be applicable to any issuance or deemed issuance of Common Stock (to the
extent provided in such more favorable antidilution provisions) which is made
(i) after the issuance of securities in connection with which such more
favorable antidilution rights are granted to the holders of Preferred Stock
pursuant to this Article 7 and (ii) at a price below the Conversion Price in
effect under the Certificate of Incorporation (or such other applicable price
which, under the terms of such more favorable antidilution rights, gives rise to
an antidilution adjustment) at the time of such issuance or deemed issuance. The
provisions of this Article 7 may be waived in any specific instance by the vote
or written consent of Stockholders holding shares of Series A Preferred Stock,
Series C Preferred Stock, Series D Preferred Stock and Series E Preferred Stock
and/or Common Stock into which such shares of Preferred Stock have been
converted, representing at least 662/3% of the voting power of such shares of
Preferred Stock (for this purpose shares of Preferred Stock shall be deemed to
have a number of votes equal to the number of shares of Common Stock into which
such shares of Preferred Stock are then convertible) and Common Stock, voting or
acting by written consent as a single class, and each Stockholder owning any
such shares of Preferred Stock or such shares of Common Stock agrees that such
Stockholder will consider in good faith any such waiver requested by the Company
as being necessary or desirable to effect a financing on terms which are
otherwise favorable to the Company.


                                    ARTICLE 8

                                   TERMINATION

                  Section 8.1 (a) Subject to subsection 8.1(c) hereof, this
Agreement shall terminate on the earlier of: (i) the date of an Initial Public
Offering or (ii) December 23, 2003.

                           (b) Notwithstanding the foregoing, this Agreement
shall in any event terminate with respect to any Stockholder when such
Stockholder no longer owns any shares of Common Stock, Preferred Stock, or other
common or preferred stock of the Company.

                           (c) The provisions set forth in Article VI, this
Section 8.1(c) and Article 9 hereof shall survive the termination of this
Agreement.


                                       30

<PAGE>

                                    ARTICLE 9

                                  MISCELLANEOUS

                  Section 9.1 Successors and Assigns. Except as otherwise
provided herein, all of the terms and provisions of this Agreement shall be
binding upon the respective successors and assigns of the parties hereto
(including any nominee of a Stockholder which holds Company Stock in its name
which is beneficially owned by such Stockholder), and shall inure to the benefit
of and shall be enforceable by the respective successors and assigns of the
parties hereto. Any breach of any of the terms or provisions of this Agreement
by a nominee of any Stockholder shall be deemed a breach of this Agreement by
such Stockholder. No Stockholder may assign any of its rights hereunder to any
Person other than a transferee of Company Stock that has complied in all
respects with the requirements of Articles 2 and 3 hereof; provided, however,
that the rights provided for in Article 4 hereof to designate a Designee shall
not be assignable. If any transferee of any Stockholder shall acquire any
securities of the Company, in any manner, whether by operation of law or
otherwise, such securities shall be held subject to all of the terms of this
Agreement, and by taking and holding such securities such Person shall be
entitled to receive the benefits of and be conclusively deemed to have agreed to
be bound by and to comply with all of the terms and provisions of this Agreement
to the extent applicable to such Person's transferor; provided, however, that
the rights provided for in Article 4 hereof to designate a Designee shall not
inure to the benefit of or apply to any such transferee. Notwithstanding the
foregoing, each transferee of any securities of the Company shall execute and
deliver to the Company a Stock Transfer Agreement in the form attached hereto
as Exhibit A.

                  Section 9.2 Amendment and Modification; Waiver of Compliance.
This Agreement may be amended only by a written instrument duly executed by the
Company and Stockholders who own a majority of the then outstanding shares of
Common Stock, and shares of Series A Preferred Stock, Series C Preferred Stock,
Series D Preferred Stock and Series E Preferred Stock and/or Common Stock into
which such shares of Preferred Stock have been converted, representing at least
66 2/3% of the voting power of such shares of Preferred Stock (for this purpose
shares of Preferred Stock shall be deemed to have a number of votes equal to the
number of shares of Common Stock into which such shares of Preferred Stock are
then convertible) and Common Stock, provided that, notwithstanding the
foregoing, without the prior consent or approval of the holders of a majority of
the Series E Preferred Stock, this Agreement may not be amended if such proposed
amendment affects the Series E Preferred Stock but does not so affect the Series
C Preferred Stock and the Series D Preferred Stock.


                                       31

<PAGE>

                           (b) Except as otherwise provided in this Agreement,
any failure of any of the parties to comply with any obligation, agreement or
condition herein may be waived by the party entitled to the benefits thereof
only by a written instrument signed by the party granting such waiver, but such
waiver or failure to insist upon strict compliance with such obligation,
agreement or condition shall not operate as a waiver of, or estoppel with
respect to, any subsequent or other failure.

                  Section 9.3 Notices. Any notice, request, claim, demand,
document and other communication hereunder to any party shall be effective upon
receipt (or refusal of receipt) and shall be in writing and delivered personally
or sent by telex, telecopy, or certified or registered mail, postage prepaid, or
other similar means of communication, as follows:

                  If to the Company or any Founder:

                           PRAECIS PHARMACEUTICALS INCORPORATED
                           One Hampshire Street
                           Cambridge, MA  02142
                           Attention:  Malcolm L. Gefter, Ph.D.
                           Telecopy: (617) 494-8414

                  with a copy to:

                           Skadden, Arps, Slate, Meagher & Flom LLP
                           One Beacon Street, 31st Floor
                           Boston, MA  02108
                           Attention:  Kent A. Coit, Esq.
                           Telecopy: (617) 573-4822

                  If to any of the Investors: at the addresses specified in
                  Schedules 1 or 2 attached hereto or a Stock Transfer
                  Agreement.

                  Section 9.4 Additional Parties. Any Person who acquires any
Company Stock from the Company after the date hereof may become a party to this
Agreement if agreed to by the Board and if such person shall agree in writing,
in form and substance reasonably acceptable to the Company, to be bound by and
to comply with all of the provisions of this Agreement.

                  Section 9.5 Entire Agreement; Governing Law. This Agreement
and the other writings referred to herein or delivered pursuant hereto, and
with respect to the Founders, the respective Stock Subscription and
Restriction Agreements dated July 27, 1993, as amended, between each of the
Founders and the Company, contain

                                       32
<PAGE>

the entire agreement among the parties hereto with respect to the subject
transactions contemplated hereby and supersede all prior oral and written
agreements and memoranda and undertakings among the parties hereto with regard
to its subject matter; provided, however, that any representations set forth in
Section 3.6 of the Original Stockholders Agreement shall survive. This Agreement
shall be governed by and construed in accordance with the laws of the State of
Delaware without regard to the conflicts of laws provisions thereof.

                  Section 9.6 Injunctive Relief. The parties hereto
acknowledge and agree that a violation of any of the terms of this Agreement
will cause the parties irreparable injury for which adequate remedy at law is
not available. Therefore, the parties agree that each party shall be entitled to
an injunction, restraining order or other equitable relief from any court of
competent jurisdiction, restraining any party from committing any violations of
the provisions of this Agreement. Without limiting the generality of the
foregoing, the Company shall have the right to any such equitable relief to
prevent any unauthorized transfers of securities of the Company hereunder.

                  Section 9.7 Expenses. Except as expressly provided herein,
each party shall pay its own expenses incident to the performance or enforcement
of this Agreement, including all fees and expenses of its counsel for activities
of such counsel undertaken pursuant to this Agreement.

                  Section 9.8 Inspection. For so long as this Agreement shall
be in effect, this Agreement shall be made available for inspection by any
Stockholder at the principal executive office of the Company.

                  Section 9.9 Headings. The 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.

                  Section 9.10 Counterparts. This Agreement may be executed in
one or more counterparts, each of which shall be deemed an original, but all of
which together shall constitute one and the same instrument.

                                       33

<PAGE>

                  IN WITNESS WHEREOF, this Agreement has been duly executed by
the requisite parties on the date first above written.


                             PRAECIS PHARMACEUTICALS
                              INCORPORATED


                             By:/s/ Kevin F. McLaughlin
                                ------------------------------------
                                Name:
                                Title: V.P. and CFO

                                /s/ Malcolm L. Gefter
                                ------------------------------------
                                Malcolm L. Gefter

                                /s/ Ethan R. Signer
                                ------------------------------------
                                Ethan R. Signer

                                /s/ A. Donny Strosberg
                                ------------------------------------
                                A. Donny Strosberg

                             MASSACHUSETTS INSTITUTE OF
                             TECHNOLOGY

                             By /s/ Allan S. Bufferd
                                ------------------------------------
                                Name: Allan S. Bufferd
                                Title: Deputy Treasurer

                                /s/ David L. Anderson
                                ------------------------------------
                                David L. Anderson

                                /s/ G. Leonard Baker, Jr.
                                ------------------------------------
                                G. Leonard Baker, Jr.

                             CANAAN VENTURES II LIMITED
                             PARTNERSHIP

                             By: Canaan Venture Partners II L.P.

                             By /s/ Harry T. Rein
                                ------------------------------------
                                Name: Harry T. Rein
                                Title: Managing General Partner

                                       34

<PAGE>

                             CANAAN VENTURES II OFFSHORE
                             C.V.

                             By: Canaan Venture Partners II L.P.

                             By /s/ Harry T. Rein
                                ------------------------------------
                                Name: Harry T. Rein
                                Title: Managing General Partner

                             CHASE VENTURE CAPITAL
                             ASSOCIATES, L.P.

                             By:      CHASE CAPITAL PARTNERS
                                      Its General Partner

                             By /s/ Damion Wicker
                                ------------------------------------
                                Name: Damion Wicker
                                Title: General Partner

                                /s/ Tench Coxe
                                ------------------------------------
                                Tench Coxe

                                /s/ Sherryl W. Hossack
                                ------------------------------------
                                James C. Gaither
                                By:      Sherryl W. Hossack
                                         Under Power of Attorney

                             GENSTAR INVESTMENT LLC

                             By /s/ Richard D. Paterson
                                ------------------------------------
                                Name: Richard D. Paterson
                                Title: Member

                             GREYLOCK LIMITED PARTNERSHIP

                             By /s/ Henry F. McCance
                                ------------------------------------
                                Name: Henry F. McCance
                                Title: General Partner

                                       35

<PAGE>

                             HIGHLAND CAPITAL PARTNERS II
                             LIMITED PARTNERSHIP

                             By:      Highland Management Partners II
                                      Limited Partnership
                                      Its General Partner

                             By /s/ Wycliffe K. Grousbeck
                                ------------------------------------
                                General Partner

                             J.H. WHITNEY & CO.

                             By /s/ William Laverack, Jr.
                                ------------------------------------
                                Name: William Laverack, Jr.
                                Title: General Partner

                             ROBERT W. AND LAURIE KITTS

                             By /s/ Robert W. Kitts
                                ------------------------------------
                                Robert W. Kitts

                             By /s/ Laurie Kitts
                                ------------------------------------
                                Laurie Kitts

                                /s/ Sherryl W. Hossack
                                ------------------------------------
                                Ronald L. Perkins
                                By:      Sherryl W. Hossack
                                         Under Power of Attorney

                             SAUNDERS HOLDINGS, L.P.

                             By /s/ G. Leonard Baker, Jr.
                                ------------------------------------
                                Name: G. Leonard Baker, Jr.
                                Title: Managing Director of the General
                                       Partner

                                       36

<PAGE>

                             SUTTER HILL VENTURES,  A CALIFORNIA LIMITED
                             PARTNERSHIP

                             By /s/ G. Leonard Baker, Jr.
                                ------------------------------------
                                Name: G. Leonard Baker, Jr.
                                Title: Managing Director of the General
                                       Partner

                             TOW PARTNERS, A CALIFORNIA LIMITED PARTNERSHIP

                             By /s/ Paul M. Wythes
                                ------------------------------------
                                Name: Paul M. Wythes
                                Title: General Partner

                             WELLS FARGO BANK, TRUSTEE
                               SHV M/P/T FBO
                             David Anderson

                             By /s/ Annik Prasad
                                ------------------------------------
                                Name: Annik Prasad
                                Title: Trust Officer

                             WELLS FARGO BANK, TRUSTEE
                                  SHV M/P/T FBO
                             G. Leonard Baker, Jr.

                             By /s/ Annik Prasad
                                ------------------------------------
                                Name: Annik Prasad
                                Title: Trust Officer

                             WHITNEY 1990 EQUITY FUND, L.P.

                             By /s/ William Laverack, Jr.
                                ------------------------------------
                                Name: William Laverack, Jr.
                                Title: General Partner

                                       37

<PAGE>

                             PAUL M. WYTHES AND MARSHA R.
                              WYTHES, TRUSTEES OF THE WYTHES
                              LIVING TRUST 7-21-87


                             By /s/ Paul M. Wythes
                                ------------------------------------
                                Name: Paul M. Wythes
                                Title: Trustee

                                /s/ William H. Younger, Jr.
                                ------------------------------------
                                William H. Younger, Jr.

                             WILLIAM H. YOUNGER, JR.,
                              TRUSTEE OF THE YOUNGER LIVING
                              TRUST 1-20-95

                             By /s/ William H. Younger, Jr.
                                ------------------------------------
                                Name: William H. Younger, Jr.
                                Title: Trustee

                                /s/ Steven P. Bird
                                ------------------------------------
                                Steven P. Bird


                                COMDISCO, INC.

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

                                /s/ Kevin J. McQuillan
                                ------------------------------------
                                Kevin J. McQuillan


                                HLM PARTNERS V, L.P.

                             By /s/ A. R. Haberkorn III
                                ------------------------------------
                                Name: A. R. Haberkorn III
                                Title: General Partner HLM Associates V,
                                       L.P. the General Partner of HLM
                                                      Partners V, L.P.

                                       38

<PAGE>

                             PHARMA/wHEALTH FUND

                             By /s/ Samuel D. Isaly
                                ------------------------------------
                                Name: Samuel D. Isaly
                                Title: Portfolio Manager

                              VULCAN VENTURES, INC.

                             By /s/ William D. Savoy
                                ------------------------------------
                                Name: William D. Savoy
                                Title: Vice President

                                /s/ Albert L. Zesiger
                                ------------------------------------
                                Albert L. Zesiger

                             ARTHUR D. LITTLE EMPLOYEE
                               PENSION PLAN

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             CITY OF STAMFORD FIREMEN'S
                             PENSION FUND

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             DEAN WITTER FOUNDATION

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             DEMVEST EQUITIES, L.P.

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                                       39

<PAGE>

                             FERRIS F. HAMILTON FAMILY TRUST

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             HBL CHARITABLE UNITRUST

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                                /s/ Albert L. Zesiger
                                ------------------------------------
                                William B. Lazar
                                By:      Albert L. Zesiger
                                         Agent & Attorney-in-fact

                                /s/ Albert L. Zesiger
                                ------------------------------------
                                Jeanne Morency
                                By:      Albert L. Zesiger
                                         Agent & Attorney-in-fact

                             WOLFSON INVESTMENT PARTNERS LP

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact


                                /s/ Albert L. Zesiger
                                ------------------------------------
                                Leonard Kingsley
                                By:      Albert L. Zesiger
                                         Agent & Attorney-in-fact


                                /s/ Albert L. Zesiger
                                ------------------------------------
                                Helen Hunt
                                By:      Albert L. Zesiger
                                         Agent & Attorney-in-fact


                                       40
<PAGE>

                             MARY ANN HAMILTON TRUST

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             MORGAN TRUST CO. OF THE
                                  BAHAMAS LTD.

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             NORWALK EMPLOYEE PENSION FUND

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             ROANOKE COLLEGE

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             STATE OF OREGON PERS/ZCG

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             TAB PRODUCTS COMPANY
                              PENSION PLAN

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             THE BREARLEY SCHOOL
                              ENDOWMENT FUND

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                                       41

<PAGE>

                             THE CHAPIN SCHOOL LTD.
                               ENDOWMENT FUND

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             THE JENIFER ALTMAN FOUNDATION

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             THE RAISER MARITAL TRUST

                             By /s/ Mark Collins, Jr.
                                ------------------------------------
                                Name: Mark Collins, Jr.
                                Title: Principal, Alex Brown Capital

                             VAN LOBEN SELS FOUNDATION

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             WARREN INVESTMENT GROUP LTD.
                               LLC

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             WARREN OTOLOGIC PROFIT
                               SHARING TRUST

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                                       42

<PAGE>

                             WELLS FAMILY LLC

                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact


                                /s/ James Finnerty
                                ------------------------------------
                                James Finnerty


                             QUANTUM INDUSTRIAL PARTNERS
                               LDC

                             By /s/ Michael C. Neus
                                ------------------------------------
                                Name: Michael C. Neus
                                Title: Attorney-In-Fact


                             TRUSTEES OF AMHERST COLLEGE


                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             CITY OF MILFORD PENSION &
                               RETIREMENT FUND


                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                             PUBLIC EMPLOYEE RETIREMENT
                               SYSTEM OF IDAHO


                             By /s/ Albert L. Zesiger
                                ------------------------------------
                                Name: Albert L. Zesiger
                                Title:   Agent & Attorney-in-fact

                                       43
<PAGE>



                                                                     SCHEDULE 1


                    NAMES AND ADDRESSES OF EXISTING INVESTORS


J.H. Whitney & Co.
177 Broad Street
Stamford, CT  06901
Attn:  William Laverack

Whitney 1990 Equity Fund, L.P.
177 Broad Street
Stamford, CT  06901
Attn:  William Laverack

Greylock Limited Partnership
One Federal Street, 26th Floor
Boston, MA  02110
Attn:  Henry F. McCance

Chase Venture Capital Associates, L.P.
c/o Chase Capital Partners
380 Madison Avenue, 12th Floor
New York, NY  10017
Attn:  Damion E. Wicker

Highland Capital Partners II Limited Partnership
Two International Place, 22nd Floor
Boston, MA  02110
Attn: Wycliffe Grousbeck

Canaan Ventures II Offshore C.V.
105 Rowayton Avenue
Rowayton, CT  06853
Attn: Harry T. Rein



<PAGE>



Canaan Ventures II Limited Partnership
105 Rowayton Avenue
Rowayton, CT  06853
Attn: Harry T. Rein

Sutter Hill Ventures
755 Page Mill Road
Suite A-200
Palo Alto, CA  94304
Attn:  G. Leonard Baker

TOW Partners
755 Page Mill Road
Suite A-200
Palo Alto, CA  94304

Saunders Holdings, L.P.
755 Page Mill Road
Suite A-200
Palo Alto, CA  94304

William H. Younger, Jr.
755 Page Mill Road
Suite A-200
Palo Alto, CA  94304

Tench Coxe
755 Page Mill Road
Suite A-200
Palo Alto, CA  94304

Ronald L. Perkins
755 Page Mill Road
Suite A-200
Palo Alto, CA  94304


                                                S1-2

<PAGE>



Wells Fargo Bank, Trustee SHV M/P/T FBO
G. Leonard Baker, Jr.
Attn: Annik Prasad
420 Montgomery Street
San Francisco, CA  94104

Wells Fargo Bank, Trustee SHV M/P/T FBO
David L. Anderson
Attn: Annik Prasad
420 Montgomery Street
San Francisco, CA  94104

James C. Gaither
755 Page Mill Road, Suite A-200
Palo Alto, CA  94304

William H. Younger, Jr., Trustee of
  The Younger Living Trust 1-20-95
755 Page Mill Road, Suite A-200
Palo Alto, CA  94304

G. Leonard Baker, Jr.
755 Page Mill Road, Suite A-200
Palo Alto, CA  94304

David L. Anderson
755 Page Mill Road, Suite A-200
Palo Alto, CA  94304

Paul M. Wythes and Marsha R. Wythes,
  Trustees of The Wythes Living
  Trust 7-21-87
755 Page Mill Road, Suite A-200
Palo Alto, CA  94304

Genstar Investment LLC
Metro Tower, Suite 1170
950 Tower Lane
Foster City, CA  94404-2121
Attn: R.D. Paterson



                                      S1-3

<PAGE>



Eric Gleacher
c/o Gleacher NatWest, Inc.
660 Madison Avenue
New York, NY  10021

Charles G. Phillips
c/o Gleacher NatWest, Inc.
660 Madison Avenue
New York, NY  10021

James Goodwin
c/o Gleacher NatWest, Inc.
660 Madison Avenue
New York, NY  10021

H. Conrad Meyer
c/o Gleacher NatWest, Inc.
660 Madison Avenue
New York, NY  10021

Emil W. Henry
c/o Gleacher NatWest, Inc.
660 Madison Avenue
New York, NY  10021

Richard A. Derbes
c/o Morgan Stanley & Co.
1585 Broadway
New York, NY  10036-8293

Robert W. Kitts
c/o Morgan Stanley & Co.
1585 Broadway
New York, NY  10036-8293

Comdisco, Inc.
6111 North River Road
Rosemont, IL  60018
Attn:  Jill C. Hanses



                                      S1-4

<PAGE>



Massachusetts Institute of Technology
Office of the Treasurer
238 Main Street
Suite 200
Cambridge, MA  02142
Attn:  Philip Rotner
       Venture Capital Officer

PHARMA/wHEALTH Fund
c/o G/A Capital Management
41 Madison Avenue
New York, NY  10010-2202
Attn:  Samuel D. Isaly

Vulcan Ventures, Inc.
110-110th Avenue Northeast
Suite 550
Bellevue, WA  98004
Attn:  William D. Savoy

Kevin J. McQuillan
345 Cervantes Road
Portola Valley, CA   94028

Steven P. Bird
8 Faxon Road
Atherton, CA  94027

HLM Partners V, L.P.
222 Berkeley Street
Boston, MA  02116
Attn:  Buck Haberkorn

Robert W. and Laurie Kitts
c/o Morgan Stanley & Co.
1585 Broadway
New York, NY  10036-8293

Albert L. Zesiger
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022



                                      S1-5

<PAGE>



Arthur D. Little Employee Pension Fund
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

City of Stamford Firemen's Pension Fund
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Dean Witter Foundation
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Demvest Equities, L.P.
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Ferris F. Hamilton Family Trust
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

HBL Charitable Unitrust
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Helen Hunt
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger



                                      S1-6

<PAGE>



Leonard Kingsley
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Mary Ann Hamilton Trust
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Morgan Trust Co. of The Bahamas Ltd.
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Norwalk Employee Pension Fund
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Roanoke College
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

State of Oregon PERS/ZCG
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Tab Products Company Pension Plan
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger



                                      S1-7

<PAGE>



The Brearley School Endowment Fund
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

The Chapin School Ltd. Endowment Fund
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

The Jennifer Altman Foundation
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

The Raiser Marital Trust
Alex Brown Capital Advisory & Trust Co.
19 South Street
Baltimore, MD  21202
Attn:  Venedia Andrews

Van Loben Sels Foundation
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Warren Investment Group Ltd. LLC
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

Warren Otologic Profit Sharing Trust
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger



                                      S1-8

<PAGE>



Wells Family LLC
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

James Finnerty
128 Cliff Road
Wellesley, MA  02181

Wolfson Investment Partners LP
c/o Zesiger Capital Group LLC
320 Park Avenue
New York,  NY  10022

William B. Lazar
c/o Zesiger Capital Group LLC
320 Park Avenue
New York,  NY  10022

Jeanne Morency
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022

Quantum Industrial Partners LDC
c/o Michael C. Neus
Attorney-in-fact
888 Seventh Avenue
New York, NY  10106


                                      S1-9

<PAGE>




                                                                     SCHEDULE 2


                      NAMES AND ADDRESSES OF NEW INVESTORS


J.H. Whitney & Co.
177 Broad Street
Stamford, CT  06901
Attn:  William Laverack

Whitney 1990 Equity Fund, L.P.
177 Broad Street
Stamford, CT  06901
Attn:  William Laverack

Greylock Limited Partnership
One Federal Street, 26th Floor
Boston, MA  02110
Attn:  Henry F. McCance

Chase Venture Capital Associates, L.P.
c/o Chase Capital Partners
380 Madison Avenue, 12th Floor
New York, NY  10017
Attn:  Damion E. Wicker

Highland Capital Partners II Limited Partnership
Two International Place, 22nd Floor
Boston, MA  02110
Attn: Wycliffe Grousbeck

Canaan Ventures II Offshore C.V.
105 Rowayton Avenue
Rowayton, CT 06853
Attn:  Harry T. Rein

Canaan Ventures II Limited Partnership
105 Rowayton Avenue
Rowayton, CT  06853
Attn: Harry T. Rein



                                      S1-1

<PAGE>



Sutter Hill Ventures, A California Limited Partnership
755 Page Mill Road
Suite A-200
Palo Alto, CA  94304
Attn:  G. Leonard Baker


Paul M. Wythes and Marsha R. Wythes,
  Trustees of The Wythes Living Trust 7-21-87
755 Page Mill Road
Suite A-200
Palo Alto, CA 94304

TOW Partners, A California Limited Partnership
755 Page Mill Road
Suite A-200
Palo Alto, CA 94304

William H. Younger,  Jr.,
  Trustee of the Younger Living Trust 1-20-95
755 Page Mill Road
Suite A-200
Palo Alto, CA 94304

Tench Coxe
755 Page Mill Road
Suite A-200
Palo Alto, CA 94304

James C. Gaither
755 Page Mill Road
Suite A-200
Palo Alto, CA 94304

Ronald L. Perkins
755 Page Mill Road
Suite A-200
Palo Alto, CA 94304

Wells Fargo Bank, Trustee
SHV M/P/T FBO David L. Anderson
420 Montogmery Street, 2nd Floor
San Francisco, CA 94104
Attn:  Annik Prasad


                                      S1-2

<PAGE>



Wells Fargo Bank, Trustee
SHV M/P/T FBO G. Leonard Baker, Jr.
420 Montgomery Street, 2nd Floor
San Francisco, CA 94104
Attn:  Annik Prasad

Genstar Investment LLC
Metro Tower, Suite 1170
950 Tower Lane
Foster City, CA 94404-2121
Attn:  R.D. Paterson

Vulcan Ventures, Inc.
110-110th Avenue Northeast
Suite 550
Bellevue, WA  98004
Attn:  William D. Savoy

HLM Partners V, L.P.
222 Berkeley Street
Boston, MA  02116
Attn:  Buck Haberkorn

Trustees of Amherst College
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY  10022
Attn:  Albert L. Zesiger

City of Milford Pension & Retirement Fund
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY 10022
Attn:  Albert L. Zesiger

Public Employee Retirement System of Idaho
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY 10022
Attn:  Albert L. Zesiger



                                      S1-3

<PAGE>



State of Oregon PERS/ZCG
c/o Zesiger Capital Group LLC
320 Park Avenue
New York, NY 10022
Attn:  Albert L. Zesiger


                                      S1-4

<PAGE>


                                                                      EXHIBIT A

                            STOCK TRANSFER AGREEMENT

         The undersigned, being the transferee of _____ shares of __________
stock1 of PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware corporation (the
"Company"), as a condition to the receipt of such shares, acknowledges that
the voting and transfer of such shares are restricted by an Amended and
Restated Stockholders Agreement dated as of April 30, 1998 (the
"Stockholders Agreement") among the Company and certain of it stockholders,
and the undersigned agrees to be bound by the terms of (and shall be a party
to) the Stockholders Agreement, as such agreement has been or may be amended,
and to be a "Stockholder" for purposes of the Stockholders Agreement.

         Agreed to this _____ day of __________, ____.


                  -----------------------------------
                  Name:

                      2
                       -------------------------------------------

Confirmed on behalf of the parties to the Stockholders Agreement as of the date
hereof.


PRAECIS PHARMACEUTICALS INCORPORATED


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



--------
1     Specify title of equity securities.
2     Include address for notices.

<PAGE>


                     AMENDMENT NO. 1 TO AMENDED AND RESTATED
                             STOCKHOLDERS AGREEMENT


                  AMENDMENT NO. 1 dated as of May 14, 1998 to Amended and
Restated Stockholders Agreement dated as of April 30, 1998 by and among PRAECIS
PHARMACEUTICALS INCORPORATED, a Delaware corporation (the "Company"), and the
stockholders of the Company referred to therein (the "Stockholders Agreement").
Capitalized terms used herein and not otherwise defined shall have the meanings
ascribed to such terms in the Stockholders Agreement.

                  The Company and the Stockholders of the Company who are
signatories hereto desire to amend the Stockholders Agreement as set forth
herein.

                  Pursuant to Section 9.2 of the Stockholders Agreement, the
Stockholders Agreement may be amended only by a written instrument duly
executed by the Company and Stockholders who own a majority of the then
outstanding shares of Common Stock, and shares of Series A Preferred Stock,
Series C Preferred Stock, Series D Preferred Stock and Series E Preferred Stock
and/or Common Stock into which such shares of Preferred Stock have been
converted, representing at least 66 2/3% of the voting power of such shares of
Preferred Stock (for this purpose shares of Preferred Stock shall be deemed to
have a number of votes equal to the number of shares of Common Stock into which
such shares of Preferred Stock are then convertible) and Common Stock.

                  The signatories hereto represent holders of greater than a
majority of the outstanding shares of Common Stock held by Stockholders and
greater than 66 2/3% of the shares of Common Stock issued or issuable upon
conversion of the Series A Preferred Stock, Series C Preferred Stock, Series D
Preferred Stock and Series E Preferred Stock.

                  In consideration of the foregoing, the parties hereto,
intending to be legally bound, hereby agree as follows:

                  1. The definition of "Related Party" set forth in Article 1 of
the Stockholders Agreement is hereby amended by (x) deleting the word
"affiliates" as it appears in the third and fifth lines of clause (iv) of such
definition and replacing it with the word "Affiliates" and (y) deleting the
period at the end of clause (iv), adding ";" to the end of such clause and
adding the following as the final clauses of such definition:

         "(v)     In the case of a limited liability company, (A) such limited
                  liability company and any of its members, (B) any former
                  members of such



<PAGE>



                  limited liability company, (C) the estates or other legal
                  representatives of any such members or former members, (D) any
                  corporation, partnership, limited liability company or other
                  business organization to which such limited liability company
                  shall sell all or substantially all of its assets or with
                  which it shall be merged and (E) any Affiliate of such limited
                  liability company;

         (vi)     In the case of a Stockholder which is, or an Affiliate of
                  which is, engaged in the investment advisory or investment
                  management business as of the date hereof (a
                  "Manager-Stockholder"), in addition to the above, any Person
                  or entity for which such Manager-Stock holder or any such
                  Affiliate acts as investment advisor or investment manager;
                  and

         (vii)    In the case of a Stockholder for which a Manager-Stockholder
                  or any Affiliate thereof acts as investment advisor or
                  investment manager, in addition to the above, any other Person
                  or entity for which such Manager-Stockholder or any such
                  Affiliate acts as investment advisor or investment manager."

                  2. Section 3.6 of the Stockholders Agreement is hereby amended
by (x) inserting "(i)" immediately before the word "unless" in the fourth line
of such section; (y) replacing the word "Purchaser" as it appears in two places
in the ninth line of such section with the word "Investor"; and (z) deleting the
period at the end of such section and adding the following as the final clause
of Section 3.6:

                  "or (ii) the Company determines that such Transfer would
                  result in the Company being required to register any of its
                  securities under the Exchange Act."

                  3. Counterparts. This Amendment No. 1 may be signed in one or
more counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument.


                                        2

<PAGE>




                  IN WITNESS WHEREOF, the parties have executed this Amendment
No. 1 to the Stockholders Agreement as of the date first written above.


                   PRAECIS PHARMACEUTICALS
                     INCORPORATED

                   By:/s/ Malcolm L. Gefter
                      --------------------------
                      Name:
                      Title:


                      /s/ Malcolm L. Gefter
                      --------------------------
                      Malcolm L. Gefter


                      /s/ Ethan R. Signer
                      --------------------------
                      Ethan R. Signer


                      /s/ A. Donna Strosberg
                      --------------------------
                      A. Donny Strosberg


                   MASSACHUSETTS INSTITUTE OF
                     TECHNOLOGY

                   By /s/ Allan S. Bufferd
                      --------------------------
                      Name: Allan S. Bufferd
                      Title: Deputy Treasurer


                      /s/ David L. Anderson
                      --------------------------
                      David L. Anderson


                      /s/ G. Leonard Baker, Jr.
                      --------------------------
                      G. Leonard Baker, Jr.



                                        3

<PAGE>



                   CANAAN VENTURES II LIMITED
                     PARTNERSHIP

                   By: Canaan Venture Partners II L.P.

                   By /s/ Harry T. Rein
                      --------------------------
                      Name:     Harry T. Rein
                      Title:    Managing General Partner


                   CANAAN VENTURES II OFFSHORE
                     C.V.

                   By: Canaan Venture Partners II L.P.

                   By /s/ Harry T. Rein
                      --------------------------
                      Name:      Harry T. Rein
                      Title:     Managing General Partner

                   CHASE VENTURE CAPITAL
                     ASSOCIATES, L.P.

                   By:      CHASE CAPITAL PARTNERS
                            Its General Partner


                   By /s/ Damion E. Wicker
                      --------------------------
                      Name:
                      Title:


                      /s/ Tench Coxe
                      --------------------------
                      Tench Coxe


                      /s/ Sherryl W. Hossack
                      --------------------------
                      James C. Gaither
                      By:      Sherryl W. Hossack
                               Under Power of Atty


                   GENSTAR INVESTMENT LLC

                   By /s/ Richard D. Paterson
                      --------------------------
                      Name:        Richard D. Paterson
                      Title:       Member



                                        4

<PAGE>



                   GREYLOCK LIMITED PARTNERSHIP

                   By /s/ Henry F. McCance
                      --------------------------
                      Name:        Henry F. McCance
                      Title:       General Partner


                   HIGHLAND CAPITAL PARTNERS II
                     LIMITED PARTNERSHIP

                   By: Highland Management Partners II
                       Limited Partnership
                       Its General Partner

                   By /s/ Wycliffe K. Grousbeck
                      --------------------------
                      General Partner


                   J.H. WHITNEY & CO.

                   By /s/ William Laverack, Jr.
                      --------------------------
                      Name:
                      Title:


                   ROBERT W. AND LAURIE KITTS

                   By /s/ Robert W. Kitts
                      --------------------------
                      Robert W. Kitts

                   By /s/ Laurie Kitts
                      --------------------------
                      Laurie Kitts


                   /s/ Sherryl W. Hossack
                      --------------------------
                      Ronald L. Perkins
                      By:      Sherryl W. Hossack
                               Under Power of Atty


                   SAUNDERS HOLDINGS, L.P.

                   By /s/ G. Leonard Baker, Jr.
                      --------------------------
                      Name:        G. Leonard Baker, Jr.
                      Title:       General Partner


                                        5

<PAGE>



                   SUTTER HILL VENTURES,  A CALIFORNIA
                   LIMITED PARTNERSHIP

                   By /s/ G. Leonard Baker, Jr.
                      --------------------------
                      Name:      G. Leonard Baker, Jr.
                      Title:     Managing Director of the
                                 General Partner


                   TOW PARTNERS, A CALIFORNIA LIMITED
                        PARTNERSHIP

                   By /s/ Paul M. Wythes
                      --------------------------
                      Name:        Paul M. Wythes
                      Title:       General Partner


                   WELLS FARGO BANK, TRUSTEE
                       SHV M/P/T FBO David Anderson

                   By /s/ Annik Prasad
                      --------------------------
                      Name:        Annik Prasad
                      Title:       Trust Officer


                   WELLS FARGO BANK, TRUSTEE
                       SHV M/P/T FBO G. Leonard Baker, Jr.

                   By /s/ Annik Prasad
                      --------------------------
                      Name:        Annik Prasad
                      Title:       Trust Officer


                   WHITNEY 1990 EQUITY FUND, L.P.

                   By /s/ William Laverack, Jr.
                      --------------------------
                      Name:
                      Title:

                   PAUL M. WYTHES AND MARSHA R.
                     WYTHES, TRUSTEES OF THE WYTHES
                     LIVING TRUST 7-21-87

                   By /s/ Paul M. Wythes
                      --------------------------
                      Name:        Paul M. Wythes
                      Title:       Trustee



                                        6

<PAGE>




                      /s/ William H. Younger, Jr.
                      --------------------------
                      William H. Younger, Jr.


                   WILLIAM H. YOUNGER, JR.,
                     TRUSTEE OF THE YOUNGER LIVING
                     TRUST 1-20-95

                   By /s/ William H. Younger, Jr.
                      --------------------------
                      Name:        William H. Younger, Jr.
                      Title:       Trustee


                      /s/ Steven P. Bird
                      --------------------------
                      Steven P. Bird


                   COMDISCO, INC.

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


                      /s/ Kevin J. McQuillan
                      --------------------------
                      Kevin J. McQuillan


                   HLM PARTNERS V, L.P.

                   By /s/ A.R. Haberkorn, III
                      --------------------------
                      Name:        A.R. Haberkorn, III
                      Title:       General Partner, HLM
                                   Associates V, L.P., the
                                   General Partner of HLM
                                   Partners V, L.P.


                   PHARMA/wHEALTH FUND

                   By /s/ Samuel D. Isaly
                      --------------------------
                      Name:        Samuel D. Isaly
                      Title:       Manager of Fund



                                       7

<PAGE>



                   VULCAN VENTURES, INC.

                   By /s/ William D. Savoy
                      -----------------------
                      Name:     William D. Savoy
                      Title:    Vice President


                   /s/ Albert L. Zesiger
                   --------------------------
                   Albert L. Zesiger


                   ARTHUR D. LITTLE EMPLOYEE
                     PENSION PLAN

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   CITY OF STAMFORD FIREMEN'S
                     PENSION FUND

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   DEAN WITTER FOUNDATION

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   DEMVEST EQUITIES, L.P.

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   FERRIS F. HAMILTON FAMILY TRUST

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact



                                        8

<PAGE>



                   HBL CHARITABLE UNITRUST

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   /s/ Albert L. Zesiger
                   --------------------------
                   William B. Lazar
                   By:      Albert L. Zesiger
                            Agent & Attorney-in-fact


                   /s/ Albert L. Zesiger
                   --------------------------
                   Jeanne Morency
                   By:      Albert L. Zesiger
                            Agent & Attorney-in-fact


                   WOLFSON INVESTMENT PARTNERS LP

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   /s/ Albert L. Zesiger
                   --------------------------
                   Leonard Kingsley
                   By:      Albert L. Zesiger
                            Agent & Attorney-in-fact


                   /s/ Albert L. Zesiger
                   --------------------------
                   Helen Hunt
                   By:      Albert L. Zesiger
                            Agent & Attorney-in-fact


                   MARY ANN HAMILTON TRUST

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact






                                       9

<PAGE>



                   MORGAN TRUST CO. OF THE
                     BAHAMAS LTD.

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   NORWALK EMPLOYEE PENSION FUND

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   ROANOKE COLLEGE

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   STATE OF OREGON PERS/ZCG

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   TAB PRODUCTS COMPANY
                     PENSION PLAN

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   THE BREARLEY SCHOOL
                     ENDOWMENT FUND

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact






                                       10

<PAGE>



                   THE CHAPIN SCHOOL LTD.
                     ENDOWMENT FUND

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   THE JENIFER ALTMAN FOUNDATION

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   THE RAISER MARITAL TRUST

                   By /s/ Mark Collins, Jr.
                      -----------------------
                      Name:     Mark Collins, Jr.
                      Title:    Principal, Alex Brown Capital


                   VAN LOBEN SELS FOUNDATION

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact



                   WARREN INVESTMENT GROUP LTD.
                     LLC

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   WARREN OTOLOGIC PROFIT
                     SHARING TRUST

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact



                                       11

<PAGE>


                   WELLS FAMILY LLC

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   /s/ James Finnerty
                   --------------------------
                   James Finnerty


                   QUANTUM INDUSTRIAL PARTNERS
                     LDC

                   By /s/ Michael C. Neus
                      -----------------------
                      Name:     Michael C. Neus
                      Title:    Attorney-In-Fact


                   TRUSTEES OF AMHERST COLLEGE

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   CITY OF MILFORD PENSION &
                     RETIREMENT FUND

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact


                   PUBLIC EMPLOYEE RETIREMENT
                     SYSTEM OF IDAHO

                   By /s/ Albert L. Zesiger
                      -----------------------
                      Name:     Albert L. Zesiger
                      Title:    Agent & Attorney-in-fact



                                       12
<PAGE>

                        AMENDMENT NO. 2 TO AMENDED AND
                        RESTATED STOCKHOLDERS AGREEMENT


                  AMENDMENT NO. 2 dated as of July 21, 1998 to Amended and
Restated Stockholders Agreement dated as of April 30, 1998 by and among
PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware corporation (the "Company"),
and the stockholders of the Company referred to therein, as amended by
Amendment No. 1 dated as of May 14, 1998 (the "Stockholders Agreement").
Capitalized terms used herein and not otherwise defined shall have the
meanings ascribed to such terms in the Stockholders Agreement.

                  The Company and the Stockholders of the Company who are
signatories hereto desire to amend the Stockholders Agreement as set forth
herein.

                  Pursuant to Article 6 of the Stockholders Agreement,
certain Stockholders have demand registration rights and all Stockholders
have piggyback registration rights.

                  The Company desires to effect an Initial Public Offering,
and, in order to facilitate such Initial Public Offering, the Company desires
that the piggyback registration rights of the Stockholders pursuant to
Section 6.2 of the Stockholders Agreement not apply to the Initial Public
Offering and the demand registration rights of the Stockholders pursuant to
Section 6.1 of the Stock holders Agreement be deferred until 180 days after
con summation of the Initial Public Offering.

                  Pursuant to Section 9.2 of the Stockholders Agreement, the
Stockholders Agreement may be amended by a written instrument duly executed
by (i) the Company; (ii) the Stockholders who own a majority of the then
outstanding shares of Common Stock, and shares of Series A Preferred Stock,
Series C Preferred Stock, Series D Preferred Stock and Series E Preferred
Stock and/or Common Stock into which such shares of Preferred Stock have been
converted, representing at least 66 2/3% of the voting power of such shares
of Preferred Stock (for this purpose shares of Preferred Stock shall be
deemed to have a number of votes equal to the number of shares of Common
Stock into which such shares of Preferred Stock are then convertible) and
Common Stock; and (iii) since such proposed amendment in part affects the
Series E Preferred Stock (and not the Series C Preferred Stock and Series D
Preferred Stock), the holders of a majority of the Series E Preferred Stock.



<PAGE>



                  In consideration of the foregoing, the parties hereto,
intending to be legally bound, hereby agree as follows:

                  1.       Deferral of Registration Rights.  The following is
hereby added as the final Section of Article 6 of the Stockholder Agreement:

                  "Section 6.7 Deferral of Registration Rights. The holders of
         Registrable Securities shall have no right (i) pursuant to Section 6.2,
         to have their Registrable Securities in cluded as part of an Initial
         Public Offering and (ii) pursuant to Section 6.1, to request that the
         Company effect a registration under the Securities Act of all or part
         of such holder's Registrable Securities until 180 days after the
         effectiveness of the registration statement for the Initial Public
         Offering; provided, however, that if the Initial Public Offering is
         not consummated on or prior to December 31, 1998, this Section 6.7
         shall be void and of no further force or effect."

                  2.       Counterparts.  This Amendment No. 2 may be signed
in one or more counterparts, each of which shall be deemed an original but
all of which together shall constitute one and the same instrument.

                  IN WITNESS WHEREOF, the parties have executed this
Amendment No. 2 to the Amended and Restated Stockholders Agreement as of the
date first written above.

                           PRAECIS PHARMACEUTICALS
                           INCORPORATED


                           By: /s/ Malcolm L. Gefter
                              -------------------------------
                              Name: Malcolm L. Gefter
                              Title: Chairman of the Board, CEO and President

                              /s/ Malcolm L. Gefter
                              --------------------------------
                              Malcolm L. Gefter



                                       2

<PAGE>




                           /s/ Ethan R. Signer
                           ------------------------------
                           Ethan R. Signer


                           /s/ A. Donny Strosberg
                           ------------------------------
                           A. Donny Strosberg


                           MASSACHUSETTS INSTITUTE OF
                             TECHNOLOGY


                           By:/s/ Allan S. Bufferd
                           ------------------------------
                           Name: Allan S. Bufferd
                           Title: Director of Investments


                           /s/ David Anderson
                           ------------------------------
                           David L. Anderson


                           /s/ G. Leonard Baker
                           ------------------------------
                           G. Leonard Baker

                           CANAAN VENTURES II LIMITED
                             PARTNERSHIP

                           By:      Canaan Venture Partners II
                                      L.P.


                           By:/s/ Harry T. Rein
                             ------------------------------
                             Name: Harry T. Rein
                             Title: General Partner


                           CANAAN VENTURES II OFFSHORE
                             C.V.

                           By:      Canaan Venture Partners II
                                     L.P.


                           By:/s/ Harry T. Rein
                             ------------------------------
                             Name: Harry T. Rein
                             Title: General Partner





                                       3

<PAGE>




                           CHASE VENTURE CAPITAL
                             ASSOCIATES, L.P.

                           By:      CHASE CAPITAL PARTNERS
                                    Its General Partner


                           By:/s/ Damion E. Wicker, M.D.
                             ------------------------------
                             Name: Damion E. Wicker, M.D.
                             Title: General Partner


                           /s/ Tench Coxe
                           --------------------------------
                           Tench Coxe


                           /s/ Sherryl W Hossack
                           --------------------------------
                           James C. Gaither
                           By:      Sherryl W. Hossack
                                    Under Power of Attorney

                           GENSTAR INVESTMENT LLC


                           By:/s/ Angus A. MacNaughton
                             ------------------------------
                             Name: Angus A. MacNaughton
                             Title: Member


                           GREYLOCK LIMITED PARTNERSHIP


                           By:/s/ Henry McCance
                             ------------------------------
                             Name: Henry McCance
                             Title: General Partner


                           HIGHLAND CAPITAL PARTNERS II
                             LIMITED PARTNERSHIP

                           By:      Highland Management Partners II
                                    Limited Partnership
                                    Its General Partner


                           By/s/ Wycliffe K. Grousbeck
                             ------------------------------
                             General Partner





                                       4

<PAGE>




                           J.H. WHITNEY & CO.


                           By:/s/ Daniel J. O'Brien
                             ------------------------------
                             Name: Daniel J. O'Brien
                             Title: General Partner


                           ROBERT W. AND LAURIE KITTS


                           By/s/ Robert W. Kitts
                             ------------------------------
                             Robert W. Kitts


                           By/s/ Laurie Kitts
                             ------------------------------
                             Laurie Kitts


                           /s/ Sherryl W. Hossack
                           --------------------------------
                           Ronald L. Perkins
                           By: Sherryl W. Hossack
                               Under Power of Atty


                           SAUNDERS HOLDINGS, L.P.


                           By:/s/ G. Leonard Baker, Jr.
                             ------------------------------
                             Name: G. Leonard Baker, Jr.
                             Title: General Partner


                           SUTTER HILL VENTURES


                           By /s/ G. Leonard Baker, Jr.
                             ------------------------------
                             Name:   G. Leonard Baker, Jr.
                             Title:  Managing Director of
                                     the General Partner


                           TOW PARTNERS


                           By /s/ Paul M. Wythes
                             ------------------------------
                             Name: Paul M. Wythes
                             Title: General Partner



                                       5

<PAGE>




                           WELLS FARGO BANK, TRUSTEE
                           SHV M/P/T FBO
                           David Anderson


                           By/s/ Vicki M. Bandel
                             ------------------------------
                             Name: Vicki M. Bandel
                             Title: A.V.P. & T.O.

                           By/s/ S. Matson
                             ------------------------------
                             Name: S. Matson
                             Title: A.V.P. & T.O.


                           WELLS FARGO BANK, TRUSTEE
                           SHV M/P/T FBO
                           G. Leonard Baker, Jr.


                           By/s/ Vicki M. Bandel
                             ------------------------------
                             Name: Vicki M. Bandel
                             Title: A.V.P. & T.O.

                           By/s/ S. Matson
                             ------------------------------
                             Name: S. Matson
                             Title: A.V.P. & T.O.


                           WHITNEY 1990 EQUITY FUND, L.P.


                           By:/s/ Daniel J. O'Brien
                             ------------------------------
                             Name: Daniel J. O'Brien
                             Title: General Partner

                           PAUL M. WYTHES AND MARSHA R.
                             WYTHES, TRUSTEES OF THE WYTHES
                             LIVING TRUST 7-21-87


                           By/s/ Paul M. Wythes
                             ------------------------------
                             Name: Paul M. Wythes
                             Title: Trustee


                           /s/ William H. Younger
                           --------------------------------
                           William H. Younger, Jr.






                                       6

<PAGE>



                           WILLIAM H. YOUNGER, JR.,
                             TRUSTEE OF THE YOUNGER LIVING
                             TRUST 1-20-95


                           By/s/ William H. Younger
                             ------------------------------
                             Name: William H. Younger, Jr.
                             Title: Trustee


                           /s/ Steven Bird
                           --------------------------------
                           Steven P. Bird


                           COMDISCO, INC.


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


                           /s/ Kevin J. McQuillan
                           --------------------------------
                           Kevin J. McQuillan


                           HLM PARTNERS V, L.P.


                           By:/s/ A.R. Haberkorn III
                             ------------------------------
                             Name: A.R. Haberkorn III
                             Title: General Partner


                           PHARMA/wHEALTH FUND


                           By:/s/ Samuel D. Isaly
                             ------------------------------
                             Name: Samuel D. Isaly
                             Title: Portfolio Manager


                           VULCAN VENTURES, INC.


                           By:/s/ William D. Savoy
                             ------------------------------
                             Name: William D. Savoy
                             Title: Vice President




                                       7

<PAGE>




                           /s/ Albert L. Zesiger
                           --------------------------------
                           Albert L. Zesiger
                           By:      Albert L. Zesiger
                                    Agent & Attorney-in-fact

                           ARTHUR D. LITTLE EMPLOYEE
                             PENSION PLAN


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           CITY OF STAMFORD FIREMEN'S
                             PENSION FUND


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           DEAN WITTER FOUNDATION


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                             /s/ Albert L. Zesiger
                             ------------------------------
                             Susan U. Halpern
                             By: Albert L. Zesiger
                                 Agent & Attorney-in-fact


                           FERRIS F. HAMILTON FAMILY TRUST


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact






                                        8

<PAGE>



                           HBL CHARITABLE UNITRUST


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           /s/ Albert L. Zesiger
                           --------------------------------
                           William B. Lazar
                           By:      Albert L. Zesiger
                                    Agent & Attorney-in-fact


                           /s/ Albert L. Zesiger
                           ------------------------------
                           Jeanne Morency
                           By: Albert L. Zesiger
                               Agent & Attorney-in-fact


                           /s/ Albert L. Zesiger
                           ------------------------------
                           Helen Hunt
                           By: Albert L. Zesiger
                               Agent & Attorney-in-fact


                           /s/ Albert L. Zesiger
                           ------------------------------
                           Leonard Kingsley
                           By: Albert L. Zesiger
                               Agent & Attorney-in-fact


                           WOLFSON INVESTMENT PARTNERS LP

                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           MARY ANN HAMILTON TRUST


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact






                                        9

<PAGE>



                           MORGAN TRUST CO. OF THE
                             BAHAMAS LTD.


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           NORWALK EMPLOYEE PENSION FUND


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           ROANOKE COLLEGE


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           STATE OF OREGON PERS/ZCG


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           TAB PRODUCTS COMPANY
                             PENSION PLAN


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact









                                       10

<PAGE>



                           THE BREARLEY SCHOOL
                             ENDOWMENT FUND


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           THE JENNIFER ALTMAN FOUNDATION


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           THE RAISER MARITAL TRUST


                           By:/s/ Mark Collins, Jr.
                             ------------------------------
                             Name:  Mark Collins, Jr.
                             Title: Principal, Alex Brown Capital


                           VAN LOBEN SELS FOUNDATION


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           WARREN INVESTMENT GROUP LTD. LLC


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact










                                       11

<PAGE>



                           WARREN OTOLOGIC PROFIT
                             SHARING TRUST


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           WELLS FAMILY LLC


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           TRUSTEES OF AMHERST COLLEGE


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           CITY OF MILFORD PENSION &
                             RETIREMENT FUND


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           PUBLIC EMPLOYEE RETIREMENT
                             SYSTEM OF IDAHO


                           By:/s/ Albert L. Zesiger
                             ------------------------------
                             Name:  Albert L. Zesiger
                             Title: Agent & Attorney-in-fact


                           /s/ James Finnerty
                           ------------------------------
                           James Finnerty




                                       12

<PAGE>


                           QUANTUM INDUSTRIAL PARTNERS LDC


                           By:/s/ Michael C. Neus
                             ------------------------------
                             Name: Michael C. Neus
                             Title: Attorney-in-fact


                                       13

<PAGE>

                         AMENDMENT NO. 3 TO AMENDED AND
                         RESTATED STOCKHOLDERS AGREEMENT

          AMENDMENT NO. 3 dated as of January 31, 2000 to Amended and Restated
Stockholders Agreement dated as of April 30, 1998 by and among PRAECIS
PHARMACEUTICALS INCORPORATED, a Delaware corporation (the "Company"), and the
stockholders of the Company referred to therein, as amended by Amendment No. 1
dated as of May 14, 1998 and Amendment No. 2 dated as of July 21, 1998 (the
"Stockholders Agreement"). Capitalized terms used herein and not otherwise
defined shall have the meanings ascribed to such terms in the Stockholders
Agreement.

          The Company and the Stockholders of the Company who are signatories
hereto desire to amend the Stockholders Agreement as set forth herein.

          Pursuant to Article 6 of the Stockholders Agreement, certain
Stockholders have demand registration rights and all Stockholders have piggyback
registration rights.

          The Company desires to effect an Initial Public Offering, and, in
order to facilitate such Initial Public Offering, the Company desires that the
piggyback registration rights of the Stockholders pursuant to Section 6.2 of the
Stockholders Agreement not apply to the Initial Public Offering and the demand
registration rights of the Stockholders pursuant to Section 6.1 of the
Stockholders Agreement be deferred until 180 days after consummation of the
Initial Public Offering.

          Pursuant to Section 9.2 of the Stockholders Agreement, the
Stockholders Agreement may be amended by a written instrument duly executed by
(i) the Company; (ii) the Stockholders who own a majority of the then
outstanding shares of Common Stock, and shares of Series A Preferred Stock,
Series C Preferred Stock, Series D Preferred Stock and Series E Preferred Stock
and/or Common Stock into which such shares of Preferred Stock have been
converted, representing at least 66 2/3% of the voting power of such shares of
Preferred Stock (for this purpose shares of Preferred Stock shall be deemed to
have a number of votes equal to the number of shares of Common Stock into which
such shares of Preferred Stock are then convertible) and Common Stock; and (iii)
in the case of certain amendments, the holders of a majority of the Series E
Preferred Stock.


                                       14
<PAGE>

          In consideration of the foregoing, the parties hereto, intending to be
legally bound, hereby agree as follows:

          1.   DEFERRAL OF REGISTRATION RIGHTS. The following is hereby added as
the final Section of Article 6 of the Stockholder Agreement:

          "Section 6.8 DEFERRAL OF REGISTRATION RIGHTS. The holders of
     Registrable Securities shall have no right (i) pursuant to
     Section 6.2, to have their Registrable Securities included as
     part of an Initial Public Offering and (ii) pursuant to Section
     6.1, to request that the Company effect a registration under the
     Securities Act of all or part of such holder's Registrable
     Securities until 180 days after the effectiveness of the
     registration statement for the Initial Public Offer ing;
     provided, however, that if the Initial Public Offering is not con
     summated on or prior to September 30, 2000, this Section 6.8
     shall be void and of no further force or effect."

          2. COUNTERPARTS. This Amendment No. 3 may be signed in one or more
counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument.


                                       15
<PAGE>

          IN WITNESS WHEREOF, the parties have executed this Amendment No. 3 to
the Amended and Restated Stockholders Agreement as of the date first written
above.

                                        PRAECIS PHARMACEUTICALS
                                          INCORPORATED

                                        By  /s/ Malcolm L. Gefter
                                            ------------------------------------
                                             Name:
                                             Title:

                                         /s/ Malcolm L. Gefter
                                        ----------------------------------------
                                        Malcolm L. Gefter

                                         /s/ Ethan R. Signer
                                        ----------------------------------------
                                        Ethan R. Signer

                                         /s/ A. Donny Strosberg
                                        ----------------------------------------
                                        A. Donny Strosberg

                                        MASSACHUSETTS INSTITUTE OF
                                        TECHNOLOGY

                                        By  /s/ Allan S. Bufferd
                                           -------------------------------------
                                            Name:    Allan S. Bufferd
                                            Title:   Treasurer

                                         /s/ David L. Anderson
                                        ----------------------------------------
                                        David L. Anderson

                                         /s/ G. Leonard Baker
                                        ----------------------------------------
                                        G. Leonard Baker



                                       16
<PAGE>

                                        CANAAN VENTURES II LIMITED
                                          PARTNERSHIP

                                        By: Canaan Venture Partners II L.P.

                                        By  /s/ Harry T. Rein
                                           -------------------------------------
                                            Name:    Harry T. Rein
                                            Title:   Managing General Partner

                                        CANAAN VENTURES II OFFSHORE
                                          C.V.

                                        By: Canaan Venture Partners II L.P.

                                        By   /s/ Harry T. Rein
                                           -------------------------------------
                                            Name:    Harry T. Rein
                                            Title:   Managing General Partner

                                        CHASE VENTURE CAPITAL
                                          ASSOCIATES, L.P.

                                        By: CHASE CAPITAL PARTNERS
                                            Its General Partner

                                        By  /s/ Damion E. Wicker, M.D.
                                           -------------------------------------
                                            Name:    Damion E. Wicker, M.D.
                                            Title:   General Partner

                                         /s/ Tench Coxe
                                        ----------------------------------------
                                        Tench Coxe



                                       17
<PAGE>

                                         /s/ Sherryl W. Hossack
                                        ----------------------------------------
                                        James C. Gaither
                                                 By Sherryl W. Hossack
                                                 Under Power of Attorney

                                        GENSTAR INVESTMENT LLC

                                        By  /s/ Richard D. Paterson
                                           -------------------------------------
                                            Name:    Richard D. Paterson
                                            Title:   Member

                                        GREYLOCK LIMITED PARTNERSHIP

                                        By  /S/ HENRY F. McCANCE
                                           -------------------------------------
                                             Name:
                                             Title:

                                        HIGHLAND CAPITAL PARTNERS II
                                          LIMITED PARTNERSHIP

                                        By: Highland Management Partners II
                                             Limited Partnership
                                               Its General Partner

                                        By  /s/ Wycliffe Grousbeck
                                           -------------------------------------
                                             General Partner

                                        J.H. WHITNEY & CO.

                                        By  /s/ Daniel J. O'Brien
                                           -------------------------------------
                                            Name:    Daniel J. O'Brien
                                            Title:   General Partner


                                       18
<PAGE>

                                        ROBERT W. AND LAURIE KITTS

                                        By  /s/ Robert W. Kitts
                                           -------------------------------------
                                             Robert W. Kitts

                                        By  /s/ Laurie Kitts
                                           -------------------------------------
                                             Laurie Kitts

                                        By  /s/ Sherryl W. Hossack
                                           -------------------------------------
                                        Ronald L. Perkins
                                                 By Sherryl W. Hossack
                                                 Under Power of Attorney

                                        SAUNDERS HOLDINGS, L.P.

                                        By  /s/ G. Leonard Baker, Jr.
                                           -------------------------------------
                                            Name:    G. Leonard Baker, Jr.
                                            Title:   General Partner

                                        SUTTER HILL VENTURES

                                        By  /s/ G. Leonard Baker, Jr.
                                           -------------------------------------
                                            Name:    G. Leonard Baker, Jr.
                                            Title:   Managing Director of the
                                                      General Partner

                                        TOW PARTNERS

                                        By  /s/ Paul M. Wythes
                                           -------------------------------------
                                            Name:    Paul M. Wythes
                                            Title:   General Partner


                                       19
<PAGE>

                                        WELLS FARGO BANK, TRUSTEE
                                        SHV M/P/T FBO
                                        David Anderson

                                        By  /s/ Vicki M. Bandel /s/ S. Matson
                                           -------------------------------------
                                            Name:    Vicki M. Bandel S. Matson
                                            Title:   AVP & TO        AVP & TO

                                        WELLS FARGO BANK, TRUSTEE
                                        SHV M/P/T FBO
                                        G. Leonard Baker, Jr.

                                        By  /s/ Vicki M. Bandel   /s/ S. Matson
                                           -------------------------------------
                                            Name:    Vicki M. Bandel S. Matson
                                            Title:   AVP & TO        AVP & TO

                                        WHITNEY 1990 EQUITY FUND, L.P.

                                        By  /s/ Daniel J. O'Brien
                                           -------------------------------------
                                            Name:    Daniel J. O'Brien
                                            Title:   General Partner

                                        PAUL M. WYTHES AND MARSHA R.
                                          WYTHES, TRUSTEES OF THE
                                          WYTHES LIVING TRUST 7-21-87

                                        By  /s/ Paul M. Wythes
                                           -------------------------------------
                                            Name:    Paul M. Wythes
                                            Title:   Trustee

                                         /s/ Sherryl W. Hossack
                                        ----------------------------------------
                                        William H. Younger, Jr.
                                                 By Sherryl W. Hossack
                                                 Under Power of Attorney


                                       20
<PAGE>

                                        WILLIAM H. YOUNGER, JR.,
                                          TRUSTEE OF THE YOUNGER LIVING
                                          TRUST 1-20-95

                                        By  /s/ Sherryl W. Hossack
                                           -------------------------------------
                                            Name:
                                            Title:
                                                 By Sherryl W. Hossack
                                                 Under Power of Attorney

                                         /s/ Steven P. Bird
                                        ----------------------------------------
                                        Steven P. Bird


                                        COMDISCO, INC.

                                        By  /s/ Jill C. Hanses
                                           -------------------------------------
                                            Name:    Jill C. Hanses
                                            Title:   Senior Vice President

                                         /s/ Kevin J. McQuillan
                                        ----------------------------------------
                                        Kevin J. McQuillan


                                        HLM PARTNERS V, L.P.
                                             By: HLM Associates V, L.P.
                                                 its General Manager

                                             By: HLM Management Co., Inc.
                                                 Managing General Partner

                                        By  /s/ A.R. Haberkorn III
                                           -------------------------------------
                                            Name:    A.R. Haberkorn III
                                            Title:   Clerk


                                       21
<PAGE>

                                        PHARMA/wHEALTH FUND

                                        By  /s/ Samuel D. Isaly
                                           -------------------------------------
                                            Name:    Samuel D. Isaly
                                            Title:   Portfolio Manager

                                        VULCAN VENTURES, INC.

                                        By  /s/ William D. Savoy
                                           -------------------------------------
                                            Name:    William D. Savoy
                                            Title:   Vice President


                                                 *
                                        ----------------------------------------
                                        Albert L. Zesiger

                                        LANDMARK SECONDARY PARTNERS, L.P.

                                        LANDMARK SECONDARY PARTNERS, L.P.

                                        By:  Landmark Partners Viii, LLC
                                                 its General Partner

                                        By:  Landmark Equity Advisors, LLC
                                                 its Managing Member

                                        By: /s/  [SIGNATURE ILLEGIBLE]
                                           -------------------------------------
                                                 Member

                                        CITY OF STAMFORD FIREMEN'S
                                          PENSION FUND

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


                                       22
<PAGE>

                                        DEAN WITTER FOUNDATION

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

                                                 *
                                        ----------------------------------------
                                        Susan U. Halpern


                                        FERRIS F. HAMILTON FAMILY TRUST

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

                                        HBL CHARITABLE UNITRUST

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

                                                 *
                                        ----------------------------------------
                                        William B. Lazar

                                                 *
                                        ----------------------------------------
                                        Jeanne Morency

                                                 *
                                        ----------------------------------------
                                        Helen Hunt

                                                 *
                                        ----------------------------------------
                                        Leonard Kingsley


                                       23
<PAGE>

                                        WOLFSON INVESTMENT PARTNERS LP

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


                                        MARY ANN HAMILTON TRUST

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

                                        MORGAN TRUST CO. OF THE
                                          BAHAMAS LTD.

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

                                        NORWALK EMPLOYEE PENSION FUND

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

                                        ROANOKE COLLEGE

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

                                        STATE OF OREGON PERS/ZCG

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


                                       24
<PAGE>

                                        TAB PRODUCTS COMPANY
                                          PENSION PLAN

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


                                        THE BREARLEY SCHOOL
                                          ENDOWMENT FUND

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

                                        THE JENIFER ALTMAN FOUNDATION

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

                                        THE RAISER MARITAL TRUST

                                        By  /s/ Mark Collins Jr.
                                        ----------------------------------------
                                            Name:    Mark Collins for
                                            Title:   Partner, Brown 1A

                                        VAN LOBEN SELS FOUNDATION

                                        By  /s/ Toni Rembe
                                           -------------------------------------
                                            Name:
                                            Title:   President

                                        WARREN INVESTMENT GROUP LTD. LLC

                                        By  /s/ Stephen R. Lippy
                                           -------------------------------------
                                            Name: Stephen R. Lippy
                                            Title: Member


                                       25
<PAGE>

                                        WARREN OTOLOGIC PROFIT
                                          SHARING TRUST

                                        By /s/ Stephen R. Lippy
                                           -------------------------------------
                                            Name: Stephen R. Lippy
                                            Title: Member

                                        WELLS FAMILY LLC

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

* -- ZESIGER CAPITAL GROUP LLC          TRUSTEES OF AMHERST COLLEGE
AGENT & ATTORNEY-IN FACT

By  /s/ Albert L. Zesiger               By       *
   -------------------------------         -------------------------------------
     Name: Albert L. Zesiger                Name:
                                            Title:

                                        CITY OF MILFORD PENSION &
                                          RETIREMENT FUND

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

                                        PUBLIC EMPLOYEE RETIREMENT
                                          SYSTEM OF IDAHO

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

                                          /s/ James Finnerty
                                        ----------------------------------------
                                        James Finnerty


                                       26
<PAGE>

                                        QUANTUM INDUSTRIAL PARTNERS LDC

                                        By  /s/ Michael C. Neus
                                           -------------------------------------
                                            Name:  Michael C. Neus
                                            Title:    Attorney-in-Fact


                                       27