<PAGE>

PP
INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE

COMMON STOCK
PAR VALUE $.01 PER SHARE

SEE REVERSE FOR CERTAIN DEFINITIONS

CUSIP 739421 10 5

This Certifies That
is the owner of

FULLY PAID AND NONASSESSABLE SHARES OF THE COMMON STOCK OF

PRAECIS PHARMACEUTICALS INCORPORATED

(hereinafter called the "Corporation") transferable on the books of the
Corporation by said owner in person or by duly authorized attorney, upon
surrender of this certificate properly endorsed. The Corporation will furnish
without charge to each stockholder who so requests, a statement of the powers,
designations, preferences and relative, participating, optional, or other
special rights of each class of stock or series thereof and the qualifications,
limitations or restrictions of such preferences and/or rights. This certificate
and the shares represented hereby are issued and shall be held subject to all
the provisions of the Certificate of Incorporation and all amendments thereto,
copies of which are on file at the office of the Transfer Agent, and the holder
hereof, by acceptance of this certificate, consents to and agrees to be bound by
all of said provisions. This certificate is not valid unless countersigned and
registered by the Transfer Agent and Registrar.
        Witness, the facsimile seal of the Corporation and by facsimile the
signatures of its duly authorized officers.

Dated:

SECRETARY
PRESIDENT
Countersigned and Registered:
AMERICAN STOCK TRANSFER & TRUST COMPANY (NEW YORK, NEW YORK) Transfer Agent and
Registrar By:

Authorized  Signature

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PRAECIS PHARMACEUTICALS INCORPORATED

The following abbreviations, when used in the inscription on the face of this
certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

TEN COM
TEN ENT
JT TEN
-D as tenants in common
-D as tenants by the entireties
-D as joint tenants with right of survivorship
   and not as tenants in common

UNIF GIFT MIN ACT-D                  Custodian
                                     (Cust)                  (Minor)
                             under Uniform Gifts to Minors
                                    Act
                                                 (State)

Additional abbreviations may also be used though not in the above list.

For Value Received                                          hereby sell, assign
 and transfer unto

          PLEASE INSERT SOCIAL SECURITY OR OTHER
           IDENTIFYING NUMBER OF ASSIGNEE







(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)



Shares
of the capital stock represented by the within certificate, and do hereby
irrevocably constitute and appoint

Attorney
to transfer the said stock on the books of the within named Corporation with
full power of substitution in the premises.
Dated

NOTICE:

The signature to this assignment must correspond with the name as written upon
the face of the certificate in every particular, without alteration or
enlargement or any change whatever.

Signature(s) Guaranteed:

THE SIGNATURE(S) SHOULD BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION
(BANKS, STOCK-

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BROKERS, SAVINGS AND LOAN ASSOCIATIONS AND CREDIT UNIONS WITH MEMBERSHIP IN AN
APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT TO S.E.C. RULE
17Ad-15.

KEEP THIS CERTIFICATE IN A SAFE PLACE. IF IT IS LOST, STOLEN, MUTILATED OR
DESTROYED, THE CORPORATION WILL REQUIRE A BOND OF INDEMNITY AS A CONDITION TO
THE ISSUANCE OF A REPLACEMENT CERTIFICATE.

This certificate also evidences and entitles the holder hereof to certain
Rights as set forth in the Rights Agreement between PRAECIS PHARMACEUTICALS
INCORPORATED (the "Company") and the Rights Agent thereunder (the "Rights
Agreement"), the terms of which are hereby incorporated herein by reference
and a copy of which is on file at the principal offices of the Company. Under
certain circumstances, as set forth in the Rights Agreement, such Rights will
be evidenced by separate certificates and will no longer be evidenced by this
certificate. The Company will mail to the holder of this certificate a copy
of the Rights Agreement, as in effect on the date of mailing, without charge,
promptly after receipt of a written request therefor. Under certain
circumstances set forth in the Rights Agreement, Rights issued to, or held
by, any Person who is, was or becomes an Acquiring Person or any Affiliate or
Associate thereof (as such terms are defined in the Rights Agreement),
whether currently held by or on behalf of such Person or by any subsequent
holder, may become null and void.