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                                                                  Exhibit 10.5

                ENVIRONMENTAL COMPLIANCE AND INDEMNITY AGREEMENT

     This environmental compliance and indemnity agreement ("Environmental
Indemnity" or "Agreement") 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 among 830 WINTER STREET LLC, a Delaware
limited liability company having an address at c/o PRAECIS PHARMACEUTICALS
INCORPORATED, One Hampshire Street, Cambridge, Massachusetts 02139 (the
"Borrower"), PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware corporation having
an address at One Hampshire Street, Cambridge, Massachusetts 02139 ("Guarantor")
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.

     As used herein:

     A. The term "Environmental Legal Requirements" shall mean all applicable
past (which have current effect), present or future federal, state, county and
local laws, by-laws, rules, regulations, codes and ordinances, or any judicial
or administrative interpretations thereof, and the requirements of any
governmental agency or authority having or claiming jurisdiction with respect
thereto, including, without limitation, all orders, decrees, judgments, rulings,
requirements, directives or notices of violation, imposed through any public or
private enforcement proceedings, that create one or more duties, obligations,
responsibilities or liabilities with respect to:

     (i)  the regulation or protection of the environment;

     (ii) the health and safety of persons and property;

     (iii)any environmental pollution, impairment or disruption;

     (iv) any environmental permits (including, without limitation, the City of
          Waltham's Board of Health Recombinant DNA Permit related to the
          Property), licenses, emissions or affluent reduction plans and
          reporting requirements; and

     (v)  in each instance further including, without limitation, laws governing
          the existence, use, storage, treatment, discharge, release,
          containment, transportation, generation, manufacture, refinement,
          handling, production, disposal or management of any Hazardous
          Materials, or otherwise regulating or providing for the protection of
          the environment, and further including, without limitation, the
          Comprehensive Environmental Response Compensation and Liability Act
          (42 U.S.C.Section 9601 ET SEQ.), the Hazardous


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          Materials Transportation Act (49 U.S.C. Section 1801 ET SEQ.), the
          Public Health Service Act (42 U.S.C. Section 300(f) ET SEQ.), the
          Pollution Prevention Act (42 U.S.C. Section 13101 ET SEQ.), the
          Federal Insecticide, Fungicide and Rodenticide Act (7 U.S.C.
          Section 136 ET SEQ.), the Resource Conservation and Recovery Act
          (42 U.S.C. Section 6901 ET SEQ.), the Federal Clean Water Act (33
          U.S.C. Section 1251 ET SEQ.), the Federal Clean Air Act (42 U.S.C.
          Section 7401 ET SEQ.), and the Massachusetts Oil and Hazardous
          Materials Release Prevention and Response Act, M.G.L. c. 21E and
          the Massachusetts Hazardous Waste Management Act, M.G.L. c. 21C.

     B. The term "Hazardous Materials" shall mean and include asbestos,
flammable materials, explosives, radioactive or nuclear substances,
polychlorinated biphenyls, other carcinogens, oil and other petroleum products,
radon gas, urea formaldehyde, chemicals, gases, solvents, pollutants or
contaminants that could be a detriment or pose a danger to the environment or to
the health or safety of any person, and any other hazardous or toxic materials,
wastes and substances which are defined, determined or identified as such in any
past (which have current effect), present or future federal, state or local
laws, by-laws, rules, regulations, codes or ordinances or any judicial or
administrative interpretation thereof.

     C. The term "Property" shall mean property known as 830 Winter Street,
Waltham, Middlesex (Southern District) County, Massachusetts as further
described on EXHIBIT A which is annexed hereto and made apart hereof, together
with all appurtenant easement areas thereto and shall also include any other
tangible property in which Lender now or hereafter receives a mortgage or
security interest in connection with the Loan.

     D. The term "Surrounding Property" shall mean any property located within
one (1) mile of the perimeter of the Property.

     E. The term "Guarantor" shall mean PRAECIS PHARMACEUTICALS INCORPORATED, a
Delaware corporation.

     F. The term "Indemnitors" shall mean, collectively, the Borrower and the
Guarantor.

     G. The term "Indemnified Party" shall mean: (i) Lender; (ii) all those
claiming by, through or under Lender, including any other present or future
holder of the Loan or any portion thereof and any present or future owner of a
participation interest therein; (iii) any subsequent owner or tenant of all or
any portion of the Property following the exercise by Lender of the rights of
Lender under the Loan Agreement, the Security Deed, the Assignment of Leases and
Rents or the other Loan Documents including, but not limited to, a foreclosure
sale or deed in lieu thereof (provided, however, with respect to this clause
(iii), the representations, warranties, covenants and indemnities contained in
this Agreement shall only relate to environmental conditions arising out of
activities and conditions on or about the Property which occurred or existed

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prior to the Lender Acquisition Date (hereinafter defined) and which were caused
by Borrower on or after the date hereof); and (iv) as to each of the foregoing,
their respective affiliate, parent and subsidiary corporations, and, as
applicable, the respective officers, directors, stockholders, agents, employees,
accountants and attorneys of any one or more of them, and any person firm or
entity which controls, is controlled by, controlling, or under common control
with, any one or more of them. Notwithstanding the foregoing, it is agreed that
the term "Indemnified Party" shall not include (i) a party unaffiliated with
Lender who purchases the Property from Lender (or Lender's nominee) following
the expiration of thirteen (13) months from the date Lender (or Lender's
nominee) first acquires the Property at foreclosure sale, deed in lieu thereof,
or otherwise ("Lender Acquisition Date") or (ii) a party unaffiliated with
Lender who purchases the Property from Lender at foreclosure sale, or any party
unaffiliated with Lender to whom Lender sells, assigns, leases or otherwise
transfers the Property prior to the expiration of the thirteen (13) month period
following the Lender Acquisition Date, if Indemnitors shall have delivered to
Lender, or Lender shall have otherwise obtained prior to sale or other
disposition of the Property to any such party, a then current environmental site
assessment report (the "Report") prepared by a licensed site professional and
complying with the provisions of Section 4.1.1 hereof, which Report does not
disclose to the reasonable satisfaction of Lender the existence of any Hazardous
Materials condition in non-compliance, or the assertion by any environmental
agency or third party of any such condition in non-compliance, with
Environmental Legal Requirements applicable to the Property or the Surrounding
Property, resulting from Hazardous Materials either (x) originating, on, under,
or from the Property or (x) originating on, under, or from any Surrounding
Property and affecting the Property, and which has not been cured to the
reasonable satisfaction of Lender.

     H. The term "Environmental Enforcement Action" shall mean all actions,
orders, requirements or liens instituted, threatened in writing, required,
completed, imposed or placed by any governmental authority and all claims made
or threatened by any other person against or with respect to the Property or any
Surrounding Property, or any present or past owner or occupant thereof, arising
out of or in connection with any of the Environmental Legal Requirements, any
environmental condition, or the assessment, monitoring, clean-up, containment,
remediation or removal of, or damages caused or alleged to be caused by, any
Hazardous Materials (i) located on or under the Property or any Surrounding
Property, (ii) emanating from the Property or any Surrounding Property, or (iii)
generated, stored, transported, utilized, disposed, managed, or released by any
Indemnitor (whether or not on, under or from the Property or the Surrounding
Property).

     I. The terms "generated, "stored, "transported, "utilized," "disposed,"
"managed," "released" and "threat of release," and all conjugates thereof, shall
have the meanings and definitions set forth in the Environmental Legal
Requirements.

     J. The term "Environmental Report" shall mean the report identified on
EXHIBIT B which is annexed hereto and made a part hereof.

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     K. All other capitalized terms used in this Agreement which are not
otherwise specifically defined herein shall have the same meaning herein as in
the Loan Agreement.

     FOR VALUE RECEIVED, and to induce Lender to grant the Loan and extend
credit to Borrower as provided for in the Loan Agreement and the other Loan
Documents, Indemnitors hereby unconditionally agree as follows:

     1.   COMPLIANCE WITH ENVIRONMENTAL LEGAL REQUIREMENTS.

          1.1  COMPLIANCE. Until such time as all Obligations with respect to
          the Loan have been paid in full, Indemnitor guaranty and agree that
          Borrower, or the other Indemnitor on behalf of Borrower, shall comply
          with all Environmental Legal Requirements applicable to Borrower
          and/or the Property and that Borrower, or the other Indemnitor on
          behalf of Borrower, shall take all remedial action necessary to avoid
          any liability of Borrower, any Lender, or any subsequent owner of the
          Property, and to avoid the imposition of, or to discharge, any liens
          on the Property, as a result of any failure by Borrower to comply with
          Environmental Legal Requirements applicable to the Property.

          1.2  PROHIBITIONS. Without limitation upon the generality of
          foregoing, Indemnitors and each of them agree that they shall not:

          (a)  except in strict compliance with Environmental Legal
          Requirements, release or permit any release or threat of release of
          any Hazardous Materials on the Property;

          (b)  except in strict compliance with Environmental Legal
          Requirements, generate or permit any Hazardous Materials to be
          generated on the Property;

          (c)  except in strict compliance with all Environmental Legal
          Requirements, store, or utilize, or permit any Hazardous Materials to
          be stored or utilized on the Property;

          (d)  dispose of or permit any Hazardous Materials to be disposed of on
          the Property; and

          (e)  fail to operate, maintain, repair and use the Property in
          accordance with Environmental Legal Requirements; or

          f)   allow, permit or suffer any other person or entity to operate,
          maintain, repair and use the Property except in accordance with
          Environmental Legal Requirements.


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     2.   NOTICE OF CONDITIONS. Except as previously disclosed to Lender in the
Environmental Report, Indemnitors shall provide Lender with prompt written
notice, but in no event later than ten (10) Business Days after obtaining any
actual knowledge or actual notice thereof, of any of the following conditions:
(i) the presence, or any release or threat of release, of any Hazardous
Materials on, under or from the Property, or any Surrounding Property, whether
or not caused by any of Indemnitors; (ii) any Environmental Enforcement Action
instituted or threatened in writing; or (iii) any condition or occurrence on the
Property, or any Surrounding Property, that may constitute or threaten to cause
to occur a violation of any of the Environmental Legal Requirements with respect
to the Property.

     3.   INDEMNITORS' AGREEMENT TO TAKE REMEDIAL ACTIONS.

          3.1  REMEDIAL ACTIONS. Upon any of Indemnitors becoming aware of the
          violation of any Environmental Legal Requirement related to the
          Property, or the presence, or any release or any threat of release, of
          any Hazardous Materials on, under, or from the Property or any
          Surrounding Property, whether or not caused by any of Indemnitors,
          Indemnitors shall, subject to the rights to contest set forth in
          Section 6, immediately take all actions: (A) to cure or eliminate any
          such violation of any such Environmental Legal Requirement relating to
          the Property and, where applicable, to arrange for the assessment,
          monitoring, clean-up, containment, removal, remediation, or
          restoration of the Property and, (B) to the extent that the presence
          of any Hazardous Materials on the Surrounding Property originated on,
          under, or from the Property, or constitutes a danger to the Property,
          take such steps as (i) are required pursuant to any Environmental
          Legal Requirements or by any governmental authority to cure or
          eliminate violation(s) of any such Environmental Legal Requirement;
          and (ii) may otherwise be advisable and reasonably requested by Lender
          consistent with Environmental Legal Requirements.

          3.2  SECURITY FOR COSTS. If the potential costs associated with the
          actions required in Section 3.1, the release of any lien against the
          Property, and the release or other satisfaction of the liability, if
          any, of any of Indemnitors with respect to the Property arising under
          or related to any of the Environmental Legal Requirements or any
          Environmental Enforcement Action are determined by Lender, in good
          faith, to exceed $500,000, Lender shall have the right to require
          Indemnitors to provide, and Indemnitors shall provide, within thirty
          (30) days after written request therefor, a bond, letter of credit or
          other similar financial assurance, in form and substance satisfactory
          to Lender, in its good faith judgment, evidencing to Lender's
          reasonable satisfaction that the necessary financial resources will be
          unconditionally available to pay for all of the foregoing.


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          3.3  ENVIRONMENTAL ASSESSMENTS. Lender shall have the right to require
          Indemnitors, at their own cost and expense, to obtain a professional
          environmental assessment of the Property in accordance with Lender's
          then standard environmental assessment requirements and sufficient in
          scope to comply with the requirements of Section 4 of this Agreement,
          upon the occurrence of any one or more of the following events: (i) an
          Event of Default; or (ii) upon receipt of any notice of any of the
          conditions specified in Section 2 of this Agreement.

     4.   LENDER'S RIGHTS TO INSPECT THE PROPERTY AND TAKE REMEDIAL ACTIONS.

          4.1  LENDER' S RIGHTS. Lender may exercise its rights and remedies
          under this Section 4 only upon and following the existence of one or
          more of the following events or conditions: (i) the Loan becomes due
          and payable in full (but has not been paid) either at its stated
          maturity or upon acceleration based upon an Event of Default; (ii) an
          Indemnified Party has taken possession of all or some portion of the
          Property based upon an Event of Default; (iii) an Indemnified Party
          has acquired title to all or some portion of the Property by virtue of
          foreclosure or deed in lieu of foreclosure; or (iv) a claim has been
          asserted against an Indemnified Party for which indemnification is
          provided in this Agreement, but none of Indemnitors has undertaken and
          is continuing to pursue commercially reasonable efforts to remediate,
          defend and otherwise indemnify any such Indemnified Party and to
          provide such Indemnified Party with reasonable assurances of such
          Indemnitor's ability, financially and otherwise, to satisfy
          Indemnitor's obligations. In any such events Lender shall have the
          right, but not the obligation, through such representatives or
          independent contractors as it may designate, to enter upon the
          Property and to expend funds to:

          4.1.1 ASSESSMENTS. Cause one or more environmental assessments of the
          Property to be undertaken, if Lender in its reasonable discretion
          determines that such assessment is appropriate. Such environmental
          assessments may include, without limitation, (i) detailed visual
          inspections of the Property, including without limitation all storage
          areas, storage tanks, drains, drywells and leaching areas; (ii) the
          taking of soils and surface and sub-surface water samples; (iii) the
          performance of soils and ground water analysis; and (iv) the
          performance of such other investigations or analysis as are necessary
          or appropriate and consistent with sound professional environmental
          engineering practice in order for Lender to obtain a complete
          assessment of the compliance of the Property and the use thereof with
          all Environmental Legal Requirements and to make a determination as to
          whether or not there is any risk of contamination (x) to the Property
          resulting from Hazardous Materials originating on, under, or from any
          Surrounding Property or (y) to any


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          Surrounding Property resulting from Hazardous Materials originating
          on, under, or from the Property;

          4.1.2 CURE. Cure any breach of the representations, warranties,
          covenants and conditions made by or imposed upon Indemnitors under
          this Agreement including, without limitation, any violation by any of
          Indemnitors, or by the Property, or by any other occupant, prior
          occupant or prior owner thereof, of any of the Environmental Legal
          Requirements applicable to the Property; provided, and on the
          condition that, none of the Indemnitors has undertaken and is
          diligently pursuing to completion the curing of any such breach in a
          manner reasonably satisfactory to Lender;

          4.1.3 PREVENTION AND PRECAUTION. Take all actions as are necessary to
          (i) prevent the migration of Hazardous Materials on, under, or from
          the Property to any other property; (ii) clean-up, contain, remediate
          or remove any Hazardous Materials on, under, or from any other
          property which Hazardous Materials originated on, under, or from the
          Property; or (iii) prevent the migration of any Hazardous Materials
          on, under, or from any other property to the Property; provided, and
          on the condition that, none of the Indemnitors has undertaken and is
          diligently pursuing to completion the taking of all such necessary
          actions in a manner reasonably satisfactory to Lender;

          4.1.4 ENVIRONMENTAL ENFORCEMENT ACTIONS. Comply with, settle, or
          otherwise satisfy any Environmental Enforcement Action including, but
          not limited to, the payment of any funds or penalties imposed by any
          governmental authority and the payment of all amounts required to
          remove any lien or threat of lien on or affecting the Property, if,
          but only if, none of the Indemnitors has undertaken and is continuing
          to pursue efforts to comply with, settle, or otherwise satisfy any
          Environmental Enforcement Action in a manner reasonably satisfactory
          to Lender; and

          4.1.5 GENERAL. Comply with, settle, or otherwise satisfy any
          Environmental Legal Requirement and correct or abate any environmental
          condition on, or which threatens, the Property and which could cause
          damage or injury to the Property or the Surrounding Property or to any
          person, if, but only if, none of the Indemnitors has undertaken and is
          continuing to pursue efforts to comply with, settle, or otherwise
          satisfy any such Environmental Legal Requirement and correct or abate
          any such environmental condition in a manner reasonably satisfactory
          to Lender.

          4.2  RECOVERY OF COSTS. Any amounts paid or advanced by Lender
          and all costs and expenditures incurred in connection with any action
          taken pursuant to the terms of this Agreement, including but not
          limited to environmental consultants' and experts' fees and expenses,
          reasonable


                                      -7-
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          attorneys' fees and expenses, court costs and all costs of assessment
          monitoring clean-up, containment, remediation, removal and
          restoration, with interest thereon at the Default Rate (as defined in
          the Note referred to in the Loan Agreement) shall be a demand
          obligation of Indemnitors to Lender and, to the extent not prohibited
          by law, (and so long as the Security Deed remains undischarged of
          record) shall be added to the Obligations secured by the Security Deed
          when paid by Lender and shall be secured by the lien of the Security
          Deed and the other Security Documents as fully and as effectively and
          with the same priority as every other obligation secured thereby.

          4.3  LENDER NOT RESPONSIBLE. The exercise by Lender of any one or more
          of the rights and remedies set forth in this Section 4 shall not
          operate or be deemed (a) to place upon Lender any responsibility for
          the operation, control, care, service, management, maintenance or
          repair of the Property or (b) make Lender the "owner" or "operator" of
          the Property or a "responsible party" within the meaning of any of the
          Environmental Legal Requirements.

          4.4  LENDER'S SUBROGATION. Furthermore, Lender, by making any such
          payment or incurring any such costs, shall be subrogated to all rights
          of each of Indemnitors or any other occupant of the Property to seek
          reimbursement from any other person including, without limitation, any
          predecessor, owner or occupant of the Property who may be a
          "responsible party" under any of the Environmental Legal Requirements
          in connection with the presence of Hazardous Materials on or under or
          which emanated from the Property.

          4.5  LENDER MAY STOP. Without limiting the generality of the other
          provisions of this Agreement, any partial exercise by Lender of any
          one or more of the rights and remedies set forth in this Section 4
          including, without limitation, any partial undertaking on the part of
          Lender to cure any failure by any of Indemnitors, or of the Property,
          or any other occupant, prior occupant or prior owner thereof, to
          comply with any of the Environmental Legal Requirements shall not
          obligate Lender to complete such actions taken or require Lender to
          expend further sums to cure such non-compliance.

     5.   INDEMNIFICATION. At all times, both before and after the repayment of
the Loan, Indemnitors hereby jointly and severally agree that they shall at
their sole cost and expense indemnify, defend, exonerate, protect and save
harmless each Indemnified Party against and from any and all damages, losses,
liabilities, obligations, penalties, claims, litigation, demands, defenses,
judgment, suits, proceedings, costs, disbursements or expenses of any kind or
nature whatsoever, including, without limitation, reasonable attorneys' and
experts' fees and disbursements, which may at any time be imposed upon,


                                      -8-
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incurred by or asserted or awarded against any Indemnified Party and arising
from or out of:

          5.1  HAZARDOUS MATERIALS. Any Hazardous Materials on, in, under or
          affecting, all or any portion of the Property, or any Surrounding
          Property on or before the date hereof, or which may hereafter affect
          all or any portion of the Property or any Surrounding Property,
          resulting from Hazardous Materials either (i) originating, on, under,
          or from the Property or (ii) originating on, under, or from any
          Surrounding Property and affecting the Property, whenever discovered;

          5.2  ENVIRONMENTAL LEGAL REQUIREMENTS. The violation of or failure to
          comply with any Environmental Enforcement Action or any Environmental
          Legal Requirement by Borrower, or with respect to the Property or any
          Surrounding Property resulting from Hazardous Materials either (i)
          originating on, under, or from the Property or (ii) originating on,
          under, or from any Surrounding Property and affecting the Property,
          existing on or before the date hereof or which may so exist in the
          future, whenever discovered;

          5.3  BREACH OF WARRANTY, REPRESENTATION OR COVENANT. Any breach of
          warranty or representation or covenant made by any Indemnitor under or
          pursuant to this Agreement; and

          5.4  GENERAL. The enforcement of this Agreement or the assertion by
          any Indemnitor of any defense to the obligations of an Indemnitor
          hereunder, whether any of such matters arise before or after
          foreclosure of the Security Deed or other taking of title to or
          possession of all or any portion of the Property by Lender or any
          other Indemnified Party, and specifically including therein, without
          limitation, the following: (i) costs incurred for any of the matters
          set forth in Section 4 of this Agreement; and (ii) costs and expenses
          incurred in ascertaining the existence or extent of any asserted
          violation of any Environmental Legal Requirements relating to the
          Property and any remedial action taken on account thereof including,
          without limitation, the costs, fees and expenses of engineers,
          geologists, chemists, other scientists, attorneys, surveyors, and
          other professionals, or testing and analyses performed in connection
          therewith.

          5.5  EXCLUSION. Notwithstanding the foregoing, it is understood and
          agreed that no Indemnified Party shall be indemnified hereunder for
          any loss (a) to the extent first caused by acts or omissions of an
          Indemnified Party subsequent to the Lender Acquisition Date or the
          date Lender takes possession of the Property pursuant to its remedial
          rights under the Loan, or (b) after repayment of the Loan and the
          satisfaction of all conditions set


                                      -9-
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          forth in Section 20 with respect to the termination of the continuing
          liability of Indemnitors under this Agreement.

     6.   RIGHT TO CONTEST. Borrower, or any other Indemnitor, may contest in
good faith any claim, demand, levy or assessment under any Environmental Legal
Requirements, including, but not limited to, any claim with respect to Hazardous
Materials, by any person or entity if:

          6.1  MATERIAL QUESTION IN GOOD FAITH. The contest is based upon a
          material question of law or fact raised by Borrower or such other
          Indemnitor in good faith;

          6.2  DILIGENT PURSUIT. Borrower or such other Indemnitor properly
          commences and thereafter diligently pursues the contest;

          6.3  NO IMPAIRMENT. The contest will not materially impair the taking
          of any required remedial action with respect to such claim, demand,
          levy or assessment;

          6.4  ADEQUATE RESOURCES. Borrower, or such other Indemnitor,
          demonstrates to Lender's reasonable satisfaction that Borrower, or
          such other Indemnitor, has the financial capability to undertake and
          pay for such contest and any remedial action then or thereafter
          necessary; and

          6.5  NO EOD. No Event of Default exists under the Loan Documents or,
          in Lender's reasonable judgment, no such Event of Default is likely to
          be precipitated by such claim, demand, levy or assessment.

     7.   WAIVERS. Indemnitors each hereby waive and relinquish to the fullest
extent now or hereafter not prohibited by applicable law:

          7.1  SURETYSHIP DEFENSES. All suretyship defenses and defenses in the
          nature thereof;

          7.2  MARSHALING. Any right or claim of right to cause a marshaling of
          Borrower's assets or of any Collateral or to cause Lender to proceed
          against any of the Collateral for the Loan before proceeding under
          this Agreement against any Indemnitor, or to require Lender to proceed
          against Indemnitors in any particular order;

          7.3  CONTRIBUTION. Until the full and Non-Contestable Payment and
          satisfaction of all Obligations, all rights and remedies against any
          other Indemnitor, including, but not limited to, any rights of
          subrogation, contribution, reimbursement, exoneration or
          indemnification pursuant to any express or implied agreement, now or
          hereafter accorded by applicable


                                      -10-
<PAGE>


          law to indemnitors, guarantors, sureties or accommodation parties.
          Provided, however, unless Lender otherwise expressly agrees in
          writing, such waiver by any particular Indemnitor shall not be
          effective to the extent that by virtue thereof such Indemnitor's
          liability under this Indemnity Agreement 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;

          7.4  NOTICE. 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 Indemnitors therefor;

          7.5  STATUTE OF LIMITATIONS. The pleading of any Statute of
          Limitations as a defense to such Indemnitor's obligations hereunder;

          7.6. JURY TRIAL. The right to a trial by jury in any matter related to
          this Environmental Indemnity; and

          EACH INDEMNITOR AND LENDER HEREBY KNOWINGLY, VOLUNTARILY AND
          INTENTIONALLY WAIVES THE RIGHT TO A TRIAL BY JURY IN RESPECT OF ANY
          LITIGATION BASED HEREON, ARISING OUT OF, UNDER OR IN CONNECTION WITH
          THIS AGREEMENT OR ANY OTHER LOAN DOCUMENTS CONTEMPLATED TO BE EXECUTED
          IN CONNECTION HEREWITH OR ANY COURSE OF CONDUCT, COURSE OF DEALINGS,
          STATEMENTS (WHETHER VERBAL OR WRITTEN) OR ACTIONS OF ANY PARTIES; THIS
          WAIVER BEING A MATERIAL INDUCEMENT FOR LENDER TO ACCEPT THIS AGREEMENT
          AND TO MAKE THE LOAN.

     8.   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 other Loan Documents. All rights and remedies of Lender shall be
cumulative and may be exercised in such manner and combination as Lender may
determine.

     9.   NO IMPAIRMENT. The liability of Indemnitors hereunder shall in no way
be limited or impaired by, and each Indemnitor hereby assents to and agrees to
be bound by, any amendment or modification of the provisions of the Loan
Documents to or with Lender and other Lenders by Borrower or any Indemnitor or
any person who succeeds Borrower as owner of the Property. In addition, the
liability of Indemnitors under this Agreement shall in no way be limited or
impaired by:


                                      -11-
<PAGE>


          9.1  EXTENSIONS. Any extensions of time for performance required by
          any of the Loan Documents;

          9.2  AMENDMENTS. Any amendment to or modification of any of the Loan
          Documents;

          9.3  TRANSFER. Any sale or assignment of or participation in the Loan
          or any portion thereof, or any sale, assignment or foreclosure of the
          Note or Security Deed or any sale or other transfer of all or part of
          the Property, except as such liability is limited by the preceding or
          succeeding provisions of this Agreement;

          9.4  EXCULPATORY LANGUAGE. 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 party;

          9.5  INACCURACIES. The accuracy or inaccuracy of any of the
          representations or warranties made by or on behalf of any Indemnitor
          under the Loan Documents or otherwise;

          9.6  RELEASE. The release of any Indemnitor, or of any other person or
          entity from performance or observance of any of the agreements,
          covenants, terms of conditions contained in any of the Loan Documents
          by operation of law, Lender's voluntary act, or otherwise;

          9.7  BANKRUPTCY OR REORGANIZATION. The filing of any bankruptcy or
          reorganization proceeding by or against any Indemnitor, any general
          partner, member or owner of any Indemnitor, Borrower, or any
          subsequent owner of the Property;

          9.8  SUBSTITUTION. The release or substitution in whole or part of any
          collateral or security for the Loan;

          9.9  FAILURE TO PERFECT. 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 to otherwise perfect, protect, secure, or
          insure any security interest or lien given as security for the Loan;

          9.10 RELEASE OF PARTIES. The release of any one or more of Indemnitors
          or any other party now or hereafter liable upon or in respect of this
          Agreement or the Loan; or


                                      -12-
<PAGE>


          9.11 INVALIDITY. The invalidity or unenforceability of all or any
          portions of any of the Loan Documents as to any Indemnitor or to any
          other person or entity.

Any of the foregoing may be accomplished with or without notice to Borrower or
any Indemnitor or with or without consideration.

     10.  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.

     11.  WARRANTIES AND REPRESENTATIONS. The Indemnitors each represent and
warrant to each Indemnified Party, the same to be true as of the date hereof, as
of the date of any request for or the making of any Advances under the Loan and
throughout the period that any of the Loan Documents shall remain in force and
effect, that except as otherwise disclosed in the Environmental Report:

          11.1 NO HAZARDOUS MATERIALS AT PROPERTY. To the best of their
          knowledge and belief, after due inquiry, except as previously
          disclosed to Lender in the Environmental Report or as permitted to
          occur in strict compliance with Environmental Legal Requirements, no
          Hazardous Materials have been or are currently generated, stored,
          transported, utilized, disposed of, managed, released or located on,
          under or from the Property, whether or not in reportable quantities,
          or in any manner introduced onto the Property including without
          limitation any drainage, septic, sewage or other waste disposal
          systems servicing the Property;

          11.2 NO VIOLATIONS CLAIMED RE PROPERTY OR INDEMNITORS. None of the
          Indemnitors has received any notice from the Massachusetts Department
          of Environmental Protection, the United States Environmental
          Protection Agency or any other governmental authority claiming that
          (i) the Property or any use thereof violates any of the Environmental
          Legal Requirements or (ii) any of the Indemnitors or any of their
          respective employees or agents have violated any of the Environmental
          Legal Requirements with respect to the Property or any Surrounding
          Property;

          11.3 NO LIABILITY TO GOVERNMENTAL AUTHORITIES. None of the Indemnitors
          has incurred any liability to the Commonwealth of Massachusetts or the
          City of Waltham or of any other governmental authority under any of
          the Environmental Legal Requirements;

          11.4 NO LIEN ON PROPERTY. No lien against the Property has arisen
          under or related to any of the Environmental Legal Requirements;


                                      -13-
<PAGE>


          11.5 NO ENFORCEMENT ACTIONS. There are no Environmental Enforcement
          Actions pending, or to the best of the Indemnitors' information,
          knowledge and belief after due inquiry, threatened in writing with
          respect to the Property.

          11.6 NO KNOWLEDGE OF HAZARDOUS MATERIALS AT SURROUNDING PROPERTY.
          Except as previously disclosed to Lender in the Environmental Report
          or as otherwise permitted to occur in strict compliance with
          Environmental Legal Requirements, none of the Indemnitors has any
          knowledge that any Hazardous Materials has been or is currently
          generated, stored, transported, utilized, disposed of, managed,
          released or located on, under or from the Surrounding Property in
          violation of or allegedly in violation of any of the Environmental
          Legal Requirements with respect to the Property;

          11.7 NO KNOWLEDGE OF VIOLATIONS RE SURROUNDING PROPERTY. Except as
          previously disclosed to Lender in the Environmental Report, none of
          the Indemnitors has any knowledge of any action or order instituted or
          threatened in writing by any person or governmental authority arising
          out of or in connection with the Environmental Legal Requirements
          involving the assessment, monitoring, cleanup, containment,
          remediation or removal of or damages caused or alleged to be caused by
          any Hazardous Materials generated, stored, transported, utilized,
          disposed of, managed, released or located on, under or from any
          Surrounding Property;

          11.8 NO UNDERGROUND STORAGE TANKS. There are no underground storage
          tanks on or under the Property;

          11.9 [Intentionally Deleted];

          11.10 VALID-AND BINDING. This Agreement constitutes the legal, valid
          and binding obligation of each of the Indemnitors in accordance with
          the respective terms hereof;

          11.11 ENTITY MATTERS. That each Indemnitor (other than one who is a
          natural person) is a duly organized validly existing entity in good
          standing under the laws of its organization and has all requisite
          power and authority to conduct its business and to own its properties
          as now conducted or owned;

          11.12 NO VIOLATIONS. The performance of the obligations evidenced
          hereby will not constitute a violation of any law, order, regulation,
          contract, organizational document or agreement to which the
          Indemnitors or any of them is a party or by which any one or more of
          them or their property is or may be bound;


                                      -14-
<PAGE>


          11.13 NO LITIGATION. There is no material litigation or administrative
          proceeding now pending or threatened in writing against the
          Indemnitors or any of them which if adversely decided could materially
          impair the ability of any one or more of the Indemnitors to pay or
          perform their respective obligations hereunder; and

          11.14 MATERIAL ECONOMIC BENEFIT. The granting of the Loan to Borrower
          will constitute a material economic benefit to each Indemnitor.


     12.  MULTIPLE COUNTERPARTS. This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original. Each of the
counterparts shall constitute but one in the same instrument and shall be
binding upon each of the parties individually as fully and completely as if all
had signed but one instrument so that the joint and several liability of each of
the Indemnitors hereunder shall be unaffected by the failure of any of the
undersigned to execute any or all of said counterparts.

     13.  NOTICES. Any notices given pursuant to this Agreement shall be
sufficient only if given in the manner provided for in the Loan Agreement.

     14.  NO ORAL CHANGE. No provision of this Agreement may be changed, waived,
discharged, or terminated orally 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.

     15.  PARTIES BOUND; BENEFIT. Except as specifically provided otherwise
herein, this Agreement shall be binding upon the Indemnitors and their
respective successors, assigns and shall be for the benefit of Lender, and of
any subsequent holder of the Loan and of any owner of a participation interest
therein. In the event the Loan is sold or transferred, then the liability of the
Indemnitors to Lender shall then be in favor of both the Lender originally named
herein and each subsequent holder of the Loan and any of interest therein. In
addition, Lender shall have the right to assign the benefits of this Agreement
to any purchaser of the Property from Lender or an affiliate of Lender in
connection with any exercise by Lender of its remedial rights under the Loan
provided, and on the condition that, any such purchaser of the Property is an
"Indemnified Party," as such term is defined in Paragraph F of the Recitals to
this Agreement.

     16.  JOINT AND SEVERAL. The obligations of each of the Indemnitors and
their respective successors, assigns, heirs and personal representatives shall
be joint and several.

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


                                      -15-
<PAGE>


     18.  GOVERNING LAW AND CONSENT TO JURISDICTION. 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 laws of the
Commonwealth of Massachusetts, without giving effect to Massachusetts principles
of conflicts of law, and, insofar as Environmental Legal Requirements and other
Environmental Legal Requirements are concerned, in accordance with applicable
federal law and Massachusetts law as well. The parties further agree that Lender
may enforce its rights under this Agreement and the other Loan Documents
including, but not limited to, the rights to sue any Indemnitor in accordance
with applicable law. The Indemnitors hereby irrevocably submit to the
nonexclusive jurisdiction of any Massachusetts State or any Federal Court
sitting in Massachusetts over any suit, action or proceeding arising out of or
relating to this Agreement and the Indemnitors hereby agree and consent 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
Massachusetts State or Federal Court sitting in Massachusetts may be made by
certified or registered mail, return receipt requested, directed to the
Indemnitors at the address indicated in Section 13 above and service so made
shall be completed five (5) days after the same shall have been so mailed.

     19. SURVIVAL. Subject to the limitations set forth in Paragraph G of the
Recitals above, and to the provisions of Section 20 below, the provisions of
this Agreement shall continue in effect and shall survive (among other events)
any payment and satisfaction of the Loan and the Obligations, any termination or
discharge of the Security Deed, foreclosure, a deed-in lieu transaction, or
release of any Collateral.

     20.  RIGHT OF INDEMNITORS TO TERMINATE OBLIGATIONS. The Indemnitors shall
have the right to terminate their continuing liability hereunder upon
fulfillment of each of the following conditions to the reasonable satisfaction
of Lender:

               (i) Lender shall have received Non-Contestable Payment in full of
          all Obligations, including but not limited to repayment in full of the
          Loan, but excluding any Obligations which might arise in the future
          under the provisions of this Agreement. The term "Non-Contestable"
          shall mean the receipt of payment of the Loan or other satisfaction of
          Obligations 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.

               (ii) The Indemnitors shall have delivered to Lender a report
          complying with the provisions of Sections 3 and 4 hereof and such
          report does not disclose the existence of any violation of
          Environmental Legal


                                      -16-
<PAGE>


          Requirements applicable to the Property, or the lawful assertion by
          any governmental agency or third party of any violation which has not
          been cured of any Environmental Legal Requirements, on or with respect
          to the Property, which report shall be dated, or last updated, to a
          date which is not earlier than the date on which the Security Deed was
          discharged or released of record.

               (iii) No Environmental Enforcement Action shall be pending or
          threatened in writing with respect to the Property.

               (iv) The Loan has been repaid without Lender or any affiliate
          thereof ever having taken actual or constructive possession of any of
          the Property, through either: (A) the appointment of a receiver, or
          (B) any other exercise of Lender's rights and remedies following an
          Event of Default under any of the Loan Documents.

               (v) No change shall have occurred in applicable Environmental
          Legal Requirements or in any official interpretation thereof
          (including governmental, judicial or administrative) which would
          result in any Indemnified Party being held responsible for causing the
          Property not to be in compliance with Environmental Legal
          Requirements.

               (vi) At least thirteen (13) months have passed since the date the
          principal, interest and fees due Lender in respect of the Loan were
          paid (without regard to the passage of time required to establish
          Non-Contestable Payment) and no claim has been asserted for which any
          indemnification is provided for in this Agreement.

     Such termination of Indemnitors' liability hereunder shall become effective
only upon the delivery by Lender to the Indemnitors of a specific written
acknowledgment of the satisfaction of the foregoing conditions and the
termination of such obligations, which Lender agrees to provide unless Lender
makes the good faith reasonable determination that the conditions to such
termination have not been satisfied.


                                      -17-
<PAGE>


       Witness the execution and delivery hereof as an instrument under seal as
of the __ day of July, 2000.


                                       INDEMNITORS:

Attest:                                PRAECIS PHARMACEUTICALS
                                       INCORPORATED,
                                       a Delaware corporation

/s/ Mary E. DeLena
-------------------------              By: /s/ Kevin F. McLaughlin
                                          -------------------------------------

                                          Name: Kevin F. McLaughlin
                                               --------------------------------
                                          Title: Senior Vice President
                                                -------------------------------
                                          Hereunto duly authorized


Attest:                                830 WINTER STREET LLC,
                                       a Delaware limited liability company

                                       By:  PRAECIS PHARMACEUTICALS
                                       INCORPORATED
                                          a Delaware Corporation,
                                          its sole member

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


                                      -18-

<PAGE>

                                   EXHIBIT A

A certain parcel of land off Winter Street, in Waltham, Middlesex County,
Massachusetts, shown as Lot 9 on Land Court Plan No. 30618E, a copy of a portion
of which is filed with the Middlesex South Registry District of the Land Court
with Certificate of Title No. 214324 in Registration Book 1201, Page 174.

Together with the benefit of rights reserved in Easement dated July 14, 1997,
filed as Document No. 1036276, and recorded in Book 27478, Page 136; as
affected by Utility Easement from owners of Lots 2 and 3 on Land Court Plan #
30618C and Lots 2, 3, B and C on Plan #669 of 1997, to Boston Edison Company
and New England Telephone and Telegraph Company, d/b/a Bell Atlantic, dated
August 27, 1998, filed as Document No. 1078157; as further affected by
Reciprocal Access and Utility Easement dated March 25, 1999, filed as
Document No. 1101665 and recorded March 26, 1999, as Instrument No. 503.

Together with the benefit of grant and reservation recited in Reciprocal Access
and Utility Easement with the owner of Lots 5 and 6 on Land Court Plan No.
30618D dated March 31, 1998, filed as Document No. 1061070, and recorded in Book
28405, page 421, affecting areas shown as "Reserved Easement Area" on a plan
entitled "Easement Plan of Land in Waltham, Massachusetts", dated March 30,
1998, recorded therewith; as affected by First Amendment to Reciprocal Access
and Utility Easement and to Reciprocal Easement Agreement dated September 10,
1998, filed as Document No. 1079645, and recorded in Book 29108, Page 346; as
further affected by Reciprocal Access and Utility Easement dated March 25, 1999,
filed as Document No. 1101665 and recorded March 26, 1999, as Instrument No.
503, and by Reciprocal Easement Agreement dated March 25, 1999, filed as
Document No. 1101666.

Together with the benefit of Reciprocal Easement Agreement with the owner of
Lots 5 and 6 on Land Court Plan No. 30618D dated March 31, 1998, filed as
Document No. 1061071, and recorded in Book 28405, Page 443, affecting areas
shown on a plan entitled "Easement Plan of Land in Waltham, Massachusetts",
dated March 30, 1998, recording therewith; as affected by First Amendment to
Reciprocal Access and Utility Easement and to Reciprocal Easement Agreement
dated September 10, 1998, filed as Document No. 1079645, and recorded in Book
29108, Page 346; as further affected by Reciprocal Easement Agreement dated
March 25, 1999, filed as Document No. 1101666.

Together with the benefit of Reciprocal Easement Agreement with the owner of Lot
8 on Land Court Plan No. 30618E dated March 10, 1999, filed as Document No.
1099963.

Together with the benefit of Reciprocal Access and Utility Easement Agreement
with the owner of Lot 8 on Land Court Plan No. 30618E dated March 10, 1999,
filed as Document No. 1099964, and recorded March 10, 1999 as Instrument No.
1121; as further affected by Reciprocal Access and Utility Easement dated
March 25, 1999, filed as Document No. 1101665 and recorded March 26, 1999, as
Instrument No. 503.

Together with the benefit of reciprocal Access and Utility Easement, dated
March 25, 1999, filed as Document No. 1101665.

Together with the benefit of Reciprocal Easement Agreement dated March 25, 1999,
filed as Document No. 1101666.

Together with the benefit of Mutual Covenants Agreement dated March 25, 1999,
filed as Document No. 1101667.

Together with the benefit of Landscape License Agreement dated March 25, 1999,
filed as Document No. 1101668.


<PAGE>

                                    EXHIBIT B


Phase I Environmental Assessment of the Vacant Land Parcel - 830 Winter Street,
Waltham Woods, Waltham, Massachusetts, dated February 1999, prepared by ENVIRON
International Corporation.

Updated Phase I Environmental Assessment of 830 Winter Street, Waltham Woods,
Waltham, Massachusetts, dated January 2000, prepared by ENVIRON International
Corporation.