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                                                                   Exhibit 10.3

                        GUARANTY OF COSTS AND COMPLETION

     This unconditional guaranty ("Guaranty") is given pursuant to the terms and
conditions of the agreement dated as of even date captioned "Acquisition and
Construction Loan Agreement" (the "Loan Agreement") by and between 830 WINTER
STREET LLC, a Delaware Limited Liability Company, having an address at c/o
PRAECIS PHARMACEUTICALS INCORPORATED, One Hampshire Street, Cambridge,
Massachusetts 02139 as "Borrower", and ANGLO IRISH BANK CORPORATION PLC
("Lender"; which term shall include any other lenders that become "Lenders" in
the future under the Loan Agreement) having an address at Stephen Court, 18-21
St. Stephen's Green, Dublin 2, Ireland. Capitalized terms used herein and not
otherwise specifically defined shall have the same meaning herein as in the Loan
Agreement.

     FOR VALUE RECEIVED, and to induce Lenders to extend credit to Borrower as
provided for in the Loan Agreement and the other Loan Documents, the
undersigned, PRAECIS PHARMACEUTICALS INCORPORATED a Delaware corporation with
its principal business address at One Hampshire Street, Cambridge, Massachusetts
02139 (the "Guarantor")," hereby unconditionally agrees as follows:

     1.   GUARANTY. Guarantor, as a primary party and not as a surety,
unconditionally and irrevocably guarantees to Lender the payment and performance
of the following "Guaranteed Obligations":

     A.   COMPLETION OF RENOVATION PROJECT. The lien free completion in
accordance with the provisions of the Loan Agreement, of the construction of the
Renovation Project, the correction of any defects or deficiencies therein, and
the prompt and full payment (and not merely the collectibility) when due of all
Project Costs payable by Borrower as necessary to achieve such completion.

     B.   OVERRUNS AND INSUFFICIENCY OF PROCEEDS. The payment by Borrower of any
amounts required to be made available to Lender under Section 2.5 (Cost
Overruns) and 10.10 (Insufficiency of Loan Proceeds) of the Loan Agreement.

         Lender may, at its option, proceed directly and at once, without
further notice, against the Guarantor hereunder, without proceeding against
Borrower, or any other person or other Collateral with respect to the Guaranteed
Obligations. Any sums payable by Guarantor hereunder shall bear interest at the
Default Rate from the date of demand until the date paid.

         This Guaranty shall survive and continue in full force and effect
beyond and after the payment and satisfaction of the Guaranteed Obligations and
the Obligations of Borrower in the event Lender is required to disgorge or
return any payment or property


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received as a result of any laws pertaining to preferences, fraudulent transfers
or fraudulent conveyances.

     2.   WAIVERS. Guarantor hereby waives and relinquishes to the fullest
extent now or hereafter not prohibited by applicable law:

          (i)  all suretyship defenses and defenses in the nature thereof;

          (ii) any right or claim of right to cause a marshaling of the assets
of Borrower or of any Collateral, or to cause Lender to proceed against any of
the other security for the Guaranteed Obligations or the Obligations of Borrower
before proceeding under this Agreement against Guarantor;

          (iii) until the satisfaction of all the Guaranteed Obligations, all
rights and remedies against Borrower, including, but not limited to, any rights
of subrogation, contribution, reimbursement, exoneration or indemnification
pursuant to any agreement, express or implied, or now or hereafter accorded by
applicable law to indemnitors, guarantors, sureties or accommodation parties;
PROVIDED, however, unless Lender otherwise expressly agrees in writing, such
waiver by Guarantor shall not be effective to the extent that by virtue thereof
the Guarantor's liability under this Guaranty or under any other Loan Document
is rendered invalid, voidable, or unenforceable under any applicable state or
federal law dealing with the recovery or avoidance of so-called preferences or
fraudulent transfers or conveyances or otherwise;

          (iv) notice of the acceptance hereof, presentment, demand for payment,
protest, notice of protest, or any and all notice of nonpayment, nonperformance,
nonobservance or default, or other proof or notice of demand whereby to charge
Guarantor therefor;

          (v)  the pleading of any Statute of Limitations as a defense to
Guarantor's obligations hereunder;

          (vi) the right to a trial by jury in any matter related to this
Guaranty; and

          (vii) the benefit of all other provisions of law which may be validly
waived.

         GUARANTOR AND LENDER MUTUALLY HEREBY KNOWINGLY, VOLUNTARILY AND
INTENTIONALLY WAIVE THE RIGHT TO A TRIAL BY JURY IN RESPECT OF ANY LITIGATION
BASED ON THIS GUARANTY, ARISING OUT OF, UNDER OR IN CONNECTION WITH THIS
GUARANTY OR ANY OF THE OTHER LOAN DOCUMENTS OR ANY COURSE OF CONDUCT, COURSE OF
DEALINGS, STATEMENTS (WHETHER VERBAL OR WRITTEN) OR ACTIONS OF ANY PARTY. THIS
WAIVER IS GIVEN AS A MATERIAL


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INDUCEMENT TO LENDER TO ACCEPT THIS GUARANTY AND TO MAKE THE LOAN.

     3.   CUMULATIVE RIGHTS. Lender's rights under this Agreement shall be in
addition to and not in limitation of all of the rights and remedies of Lender
under the Loan Documents. All rights and remedies of Lender shall be cumulative
and may be exercised, in accordance with the Loan Agreement, in such manner and
combination as Lender may determine.

     4.   NO IMPAIRMENT. The liability of Guarantor hereunder shall in no way be
limited or impaired by, and Guarantor hereby assents to and agrees to be bound
by, any amendment or modification of the provisions of the Loan Documents,
whenever occurring, but no such amendment or modification shall operate so as to
increase the scope of Guarantor's liability hereunder unless such amendment or
modification is assented to by the Guarantor. In addition, the liability of
Guarantor under this Guaranty and the other Loan Documents shall in no way be
limited or impaired by:

          (i)  any extensions of time for performance required by any of the
Loan Documents;

          (ii) any amendment to or modification of any of the Loan Documents
except that no such amendment or modification made without the consent of
Guarantor shall increase the scope or amount of Guarantor's liability hereunder;

          (iii) any sale or assignment of or participation in the Loan or any
portion thereof or any sale, assignment or foreclosure of the Security Deed or
any sale, transfer or exchange of all or part of the Site and Improvements;

          (iv) any exculpatory, or nonrecourse, or limited recourse, provision
in any of the Loan Documents limiting Lender's recourse to the property
encumbered by the Security Deed or to any other property or limiting Lender's
rights to a deficiency judgment against Borrower or any other person or entity;

          (v)  the accuracy or inaccuracy of any of the representations or
warranties made by or on behalf of Borrower, any member or partner of Borrower,
or Guarantor, under any Loan Document or otherwise;

          (vi) the release of Borrower, any member or partner of Borrower, or
any other person or entity, from performance or observance of any of the
agreements, covenants, terms or conditions contained in any of the Loan
Documents by operation of law, Lender's voluntary act, or otherwise;

          (vii) the filing of any bankruptcy or reorganization proceeding by or
against Borrower, any member or partner of Borrower, or any subsequent owner of
the Site and Improvements;


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          (viii) the release or substitution in whole or part of any collateral
or security for the Obligations or the Guaranteed Obligations;

          (ix) Lender's failure to record the Security Deed or file any UCC
financing statements, or Lender's improper recording or filing of any thereof,
or Lender's failure to otherwise perfect, protect, secure, or insure any
security interest or lien given as security for the Obligations;

          (x)  the release of any other party now or hereafter liable upon or in
respect of this Guaranty or any of the other Loan Documents; or

          (xi) the invalidity or unenforceability of all or any portion of any
of the Loan Documents as to Borrower or any other person or entity.

          Any of the foregoing may be accomplished with or without notice to
Borrower, any member or partner of Borrower or Guarantor and with or without
consideration.

     5.   DELAY NOT WAIVER. No delay on Lender's part in exercising any right,
power or privilege hereunder or under any of the Loan Documents shall operate as
a waiver of any such privilege, power or right. No waiver by Lender in any
instance shall constitute a waiver in any other instance.

     6.   WARRANTIES AND REPRESENTATIONS. Guarantor warrants and represents to
Lender for the express purpose of inducing Lender to enter into the Loan
Agreement, as follows:


          (i)  NO VIOLATION. The payment and performance by Guarantor of its
obligations under this Guaranty do not and shall not constitute a violation of
any law, order, regulation, contract or agreement to which Guarantor is a party
or by which any Guarantor or any Guarantor's property may be bound;

          (ii) NO LITIGATION. There is no material litigation now pending or, to
the best of Guarantor's knowledge threatened in writing, against Guarantor
which, if adversely decided and not otherwise covered by adequate insurance
would materially impair the ability of Guarantor to pay and perform Guarantor's
obligations under this Guaranty. There is no litigation (whether or not
material) pending, or, to the best of Guarantor's knowledge threatened in
writing, against Guarantor in which the amount in controversy exceeds $25,000.00
which is not covered by adequate insurance, and which has not been previously
disclosed in writing to Lender together with a written explanation of why such
litigation is not material;

          (iii) ENTITY MATTERS. Guarantor is a duly organized, validly existing
corporation, organized and in good standing under the laws of Delaware, has all
requisite


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power and authority to conduct its business and to own its property as
now conducted or owned, and is qualified to do business in all jurisdictions
where the nature and extent of its business is such that such qualification is
required by law;

          (iv) VALID AND BINDING. This Guaranty constitutes Guarantor's legal,
valid and binding obligation in accordance with the respective terms hereof,
subject to bankruptcy, insolvency and similar laws of general application
affecting the rights and remedies of creditors and with respect to the
availability of remedies of specific enforcement subject to the discretion of
the court before which proceedings therefor may be brought;

          (v)  SOLVENCY. Guarantor is solvent and is not rendered insolvent by
the obligations undertaken in this Guaranty. Guarantor is not contemplating
either the filing of a petition or proceeding under any state or federal
bankruptcy or insolvency or reorganization laws or the liquidating of all or a
major portion of Guarantor's property, and Guarantor has no knowledge of any
such petition or proceeding being filed or threatened in writing against any
Guarantor;

          (vi) FINANCIAL INFORMATION. True and complete copies of the financial
statements of Guarantor have been delivered to Lender and each of the same are
true, accurate and complete and fairly present Guarantor's financial condition
as of the dates thereof and no material and adverse change has occurred in
Guarantor's financial condition or business since the respective dates thereof;
and each financial statement of Guarantor submitted in the future shall be true,
accurate and complete and shall fairly present Guarantor's financial condition
as of the dates thereof;

          (vii) MATERIAL ECONOMIC BENEFIT. The granting of the Loan to Borrower
will constitute a material economic benefit to Guarantor; and

          (viii) GENERAL. Guarantor has disclosed all material facts and
conditions to Lender which are necessary to make the representations and
warranties of Guarantor set forth herein not materially misleading.

     7.   NOTICES. Any notice or other communication in connection with this
Guaranty shall be delivered in accordance with Section 22 of the Loan Agreement,
addressed to Guarantor in care of the address for Borrower set forth therein.

     8.   NO ORAL CHANGE. No provision of this Agreement may be changed, waived,
discharged, or terminated orally (in person or by telephone) or by any other
means except by an instrument in writing signed by the party against whom
enforcement of the change, waiver or discharge or termination is sought.

     9.   PARTIES BOUND; BENEFIT. This Agreement shall be binding upon Guarantor
and Guarantor's successors, assigns, heirs and personal representatives and
shall be for the benefit of Lender, and of any subsequent holder of all or a
part of Lender's interest in


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the Loan and of any owner of a participation interest therein. In the event the
interest of Lender under the Loan Documents is sold or transferred in whole or
in part, then the liability of the Guarantor to such Lender(s) shall then be in
favor of both the Lender originally named herein and each subsequent holder of
Lender's interest therein, to the extent of their respective interests. In
connection with any such sale, assignment or participation, Lender shall have
the right to disclose to, or share with, any actual or potential transferee or
participant all information, including, but not limited to, financial
information in Lender's possession regarding the Loan, Borrower, Guarantor or
the Site and Improvements.

     10.  PARTIAL INVALIDITY. Each of the provisions hereof shall be
enforceable against Guarantor to the fullest extent now or hereafter not
prohibited by applicable law. The invalidity or unenforceability of any
provision hereof shall not limit the validity or enforceability of each other
provision hereof.

     11.  GOVERNING LAW. This Agreement and the rights and obligations of the
parties hereunder shall in all respects be governed by and construed and
enforced in accordance with the internal laws of the Commonwealth of
Massachusetts without giving effect to principles of conflicts of law. It is
understood and agreed that this Guaranty and all of the other Loan Documents
were negotiated, executed and delivered in the Commonwealth of Massachusetts,
which Commonwealth the parties agree has a substantial relationship to the
parties and to the underlying transactions embodied by the Loan Documents.

     12.  CONSENT TO JURISDICTION. Guarantor hereby irrevocably submits to the
nonexclusive personal jurisdiction of any Massachusetts State Court or any
Federal Court sitting in Massachusetts over any suit, action or proceeding
arising out of or relating to this Guaranty. Guarantor hereby agrees and
consents that in addition to any methods of service of process provided for
under applicable law, all service of process in any such suit, action or
proceeding in any sitting in Massachusetts State or Federal Court may be made by
certified or registered mail, return receipt requested, directed to Guarantors
at the addresses indicated in Section 7 above and service so made shall be
deemed completed five (5) days after the same shall have been so mailed.

     13.  FINANCIAL STATEMENTS AND REPORTS. Guarantor shall furnish or cause to
be furnished to Lender from time to time the following financial statements and
reports and other information:

          (i)  No later than ninety (90) days after the end of each fiscal year,
audited, annual financial statements of Guarantor, as of the end of Guarantor's
most recent fiscal year prepared in accordance with generally accepted
accounting principles consistently applied by Ernst & Young LLP or by an
independent certified public accountant reasonably acceptable to Lender and
certified, without qualification, by such accountant, each such financial
statement to include a statement of all contingent


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liabilities and to be supplemented by such detail and supporting data and
schedules as Lender may from time to time reasonably determine;

          (ii) Within forty-five (45) days following the end of each of the
first three (3) fiscal quarters, internally prepared financial statements of
Guarantor as of the end of its most recent quarter, such financial statements to
be prepared in accordance with generally accepted accounting principles,
consistently applied, and certified by the principal financial officer of
Guarantor to be true, accurate and complete, all in such form and manner of
presentation as Lender may reasonably require;

          (iii) Promptly upon becoming aware thereof, written notice from
Guarantor of any event or condition which might have a materially adverse effect
on the business, operations, assets, condition (financial or otherwise) or
prospects of Guarantor or the ability of Guarantor to perform under this
Guaranty (including but not limited to, litigation commenced or threatened in
writing against Guarantor, judgments rendered against Guarantor, liens filed
against any property of Guarantor, defaults claimed under indebtedness for
borrowed money for which Guarantor is primarily or secondarily liable, or
bankruptcy, insolvency or trustee or receivership proceedings commenced against
Guarantor) such notice to specify the nature and the period of existence of such
event or condition, the anticipated effect thereof, and what action Guarantor is
taking or proposes to take with respect thereto;

          (iv) Such other financial information as Lender may reasonably require
from time to time.

     14.  ADDITIONAL COVENANTS OF THE GUARANTOR. Guarantor covenants and agrees
as follows:

          14.1. GENERAL. Guarantor shall pay, perform, observe and comply with
all of the obligations, terms, covenants and conditions set forth in this
Guaranty and in a certain Guaranty of Non-Recourse Exceptions of even date
herewith from Guarantor to Lender. Guarantor shall not suffer or permit to occur
any material adverse change in its financial condition, liquidity or business
from that shown on or reflected in its financial statements most recently
submitted to Lender prior to the date hereof.

          14.2. OWNERSHIP AND CONTROL OF GUARANTOR. Except as otherwise
permitted under Section 11.7 of the Loan Agreement or in connection with an
acquisition by Guarantor, Guarantor shall not permit any change in the ownership
or control of Guarantor; provided, however that a change of control shall not be
deemed to include changes in the Board of Directors of Guarantor.

          14.3. LEGAL EXISTENCE. Guarantor will do or cause to be done all
things necessary to preserve and keep in full force and effect its legal
existence, and to comply with all applicable laws and regulations with respect
to which non-compliance would


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cause a materially adverse impact on the Site, Improvements, Loan, Borrower or
Guarantor (including, without limitation, ERISA, tax, securities and
environmental laws).

          14.4. MERGERS. Except otherwise permitted under Section 11.7 of the
Loan Agreement, Guarantor will not become a party to any merger or
consolidation, or agree to effect any asset acquisition or stock acquisition
(other than the acquisition of assets in the ordinary course of business).

          14.5. TRANSFER OF INTEREST. Except as otherwise permitted under
Section 11.7 of the Loan Agreement, Guarantor will not make or permit to be
made, by voluntary or involuntary means, any transfer or encumbrance of any of
its direct or indirect interests in Borrower, or any dilution of its direct or
indirect interests in Borrower. In no event shall any transfer of interest
permitted under the Loan Agreement affect the liability of Guarantor hereunder.

     15.  LEGAL FEES, COSTS AND EXPENSES. Guarantor further agrees to pay upon
demand all costs and expenses reasonably incurred by Lender or its successors or
assigns in connection with enforcing any of the rights or remedies of Lender, or
such successors or assigns, under or with respect to this Guaranty including,
but not limited to, reasonable attorneys' fees and the out-of-pocket expenses
and disbursements of such attorneys. Any such amounts which are not paid within
fifteen (15) days of demand therefor shall bear interest at the Default Rate
from the date of demand until paid.

     16.  SUBORDINATION.

          16.1. Any indebtedness of Borrower to Guarantor, or to any affiliated
entity of Guarantor, now or hereafter existing together with any interest
thereon shall be, and such indebtedness is, hereby deferred, postponed and
subordinated to the prior, full and Non-Contestable Payment and satisfaction of
all Obligations of Borrower under the Loan Agreement, the Note, the Security
Deed and the other Loan Documents. Payment and satisfaction of Obligations shall
be deemed "Non-Contestable Payment" only upon such payment and satisfaction and
the expiration of all periods of time within which a claim for the recovery of a
preferential payment, or fraudulent conveyance, or fraudulent transfer, in
respect of payments received by Lender as to the Obligations could be filed or
asserted with: (A) no such claim having been filed or asserted, or (B) if so
filed or asserted, the final, non-appealable decision of a court of competent
jurisdiction denying the claim or assertion.

          16.2 At all times until the full and Non-Contestable Payment and
satisfaction of the Obligations of Borrower with respect to the Loan (and
including interest accruing on the Note after the commencement of a case by or
against Borrower under the Bankruptcy Code now or hereafter in effect, which
interest the parties agree shall remain a claim that is prior and superior to
any claim of Guarantor or any affiliated entity of Guarantor notwithstanding any
contrary practice, custom or ruling in cases under the Bankruptcy Code, as now
or hereafter in effect, generally), Guarantor agrees


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and to cause each of its affiliated entities to agree not to accept any payment
or satisfaction for any kind of indebtedness of Borrower to Guarantor, or any
affiliated entity, and hereby assigns such indebtedness to Lender including, but
not limited to, the right to file proofs of claim and to vote thereon in
connection with any such case under the Bankruptcy Code, as now or hereafter in
effect, and the right to vote on any plan of reorganization.

          16.3 Any mortgage, security interest, lien or charge on the
Collateral, all rights therein and thereto, and on the revenue and income to be
realized therefrom, which Guarantor, or any affiliated entity, may have or
obtain as security for any loans, advances, indebtedness or costs shall be, and
such mortgage security interest, lien or charge hereby is, subordinated to the
Security Deed and to the full and Non-Contestable Payment and satisfaction of
all Obligations of Borrower under the Loan Agreement, the Note and the other
Loan Documents.

          16.4 In addition to the foregoing, and not in limitation thereof, any
claims of Guarantor, or any affiliated entity of Guarantor, of subrogation,
contribution, reimbursement, exoneration, indemnification, or reimbursement
arising out of any payment made on this Guaranty, whether such claim is based
upon an express or implied contract, or operation of law, are hereby waived
until the full and Non-Contestable Payment and satisfaction of all Obligations
of Borrower under the Loan Documents. Provided, however, unless Lender otherwise
expressly agrees in writing, such waiver by any particular Guarantor shall not
be effective to the extent that by virtue thereof such Guarantor's liability
under this Guaranty or under any other Loan Document is rendered invalid,
voidable, or unenforceable under the applicable state or federal law dealing
with the recovery or avoidance of so-called preferences or fraudulent
conveyances or otherwise.

     17.  COUNTERPARTS. This Guaranty may be executed in several counterparts,
each of which when executed and delivered is an original, but all of which
together shall constitute one instrument. In making proof of this Guaranty, it
shall not be necessary to produce or account for more than one such counterpart
which is executed by the party against whom enforcement of such Guaranty is
sought.


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     Witness the execution and delivery hereof, under seal, as of the 11th
day of July, 2000.

                                       GUARANTOR:

Attest:                                PRAECIS PHARMACEUTICALS
                                       INCORPORATED, a Delaware corporation

/s/ Mary E. DeLena                     By: /s/ Kevin F. McLaughlin
-----------------------                   ------------------------------------
                                          Name: Kevin F. McLaughlin
                                               -------------------------------
                                          Its: Senior Vice President
                                               -------------------------------
                                          Hereunto duly authorized


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