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                                                                    EXHIBIT 10.2

                                                                  EXECUTION COPY

               AMENDMENT NO. 1 TO ASSIGNMENT AND LICENSE AGREEMENT

     THIS AMENDMENT NO. 1 TO ASSIGNMENT AND LICENSE AGREEMENT (this "Amendment")
dated this 1st day of July, 2003 (the "Effective Date"), is entered into between
Cubist Pharmaceuticals Incorporated, a Delaware corporation having its principal
place of business at 65 Hayden Avenue, Lexington, MA 02421 and its Affiliates
(collectively "CUBIST") and Eli Lilly and Company, an Indiana corporation having
an office and principal place of business at Lilly Corporate Center,
Indianapolis, Indiana 46285 and its Affiliates ("LILLY").

                                   WITNESSETH:

     WHEREAS, the parties entered into an ASSIGNMENT AND LICENSE AGREEMENT
effective as of October 20, 2000 (the "Agreement").

     WHEREAS, the parties have entered into a STOCK TRANSFER AGREEMENT and a
REGISTRATION RIGHTS AGREEMENT on even date herewith whereby CUBIST will transfer
to LILLY eight million dollars ($8,000,000) worth of shares of CUBIST common
stock (the "Shares") and cause such Shares to be registered under applicable
securities laws in return for a reduction of one percent (1%) on all royalties
to be paid under the Agreement.

     WHEREAS, the parties desire to amend the Agreement in accordance with
Section 17.04 thereof in certain respects as set forth below.

     NOW, THEREFORE, in consideration of the foregoing premises and the mutual
covenants set forth below, the parties amend the Agreement and agree as follows:

                                    AMENDMENT

     The Parties have reached mutual agreement that as of the Amendment
Effective Date (as defined below), each of the specific royalty rates set forth
in Sections 4.01, 4.02 and 4.03 of the Agreement, shall be deleted and shall be
replaced with a specific royalty rate that is equal to (a) the previous royalty
rate MINUS (b) one percentage point.

                            AMENDMENT EFFECTIVE DATE

     The amendments referred to above shall become effective upon the earlier
of: (a) the date on which the Registration Statement contemplated by the
REGISTRATION RIGHTS AGREEMENT shall have been declared effective by the
Securities and Exchange Commission; (b) the date on which Lilly makes its first
sale of any Shares, which date shall be provided to Cubist in writing within
five (5) days of such sale, provided that Lilly's failure to notify Cubist
thereof shall not impact the determination of the Amendment Effective Date for
purposes of this clause (b); or (c) one (1) year from the date of this Amendment
(the date on which the amendments become effective being the "Amendment
Effective Date"). In the event the Closing contemplated by the STOCK TRANSFER
AGREEMENT does not occur on July 21, 2003, this Amendment shall become null and
void.

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                                  MISCELLANEOUS

     1.1  DEFINED TERMS. All terms used, but not defined, herein shall have the
respective meanings set forth in the Agreement.

     1.2  CONTINUING EFFECT. This Amendment shall be effective for all purposes
as of the Amendment Effective Date. Except as otherwise expressly modified by
this Amendment, the Agreement shall remain in full force and effect in
accordance with its terms.

     1.3  COUNTERPARTS. This Amendment may be executed in two 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 as of the date
first written above.

ELI LILLY AND COMPANY

By: /s/ Charles E. Golden
    ------------------------------------------

Name:  Charles E. Golden

Title: Executive Vice President and
       Chief Financial Officer

CUBIST PHARMACEUTICALS, INC.

By: /s/ Oliver S. Fetzer
    -------------------------------------------

Name:   Oliver S. Fetzer
      -----------------------------------------

Title:  Senior Vice President, Corporate
        Development and Chief Business Officer
       ----------------------------------------

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                                                                    EXHIBIT 10.3

                                CREDIT AGREEMENT

     This CREDIT AGREEMENT, dated as of April 30, 2003, by and between CUBIST
PHARMACEUTICALS, INC., a Delaware corporation (the "BORROWER") and CITIZENS BANK
OF MASSACHUSETTS, a Massachusetts bank (the "LENDER").

     In consideration of the mutual covenants contained herein, and for other
good and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

SECTION 1.   DEFINITIONS.

     All capitalized terms not defined herein but defined in APPENDIX A attached
hereto shall have the meanings given to such terms in APPENDIX A attached
hereto. All terms defined in this Agreement shall also have such defined
meanings when used in the other Financing Documents or any certificate or other
document made or delivered pursuant hereto or thereto, unless otherwise
specified herein or therein.

     All references in this Agreement to Sections, Subsections, Exhibits,
Schedules and Appendices refer to the Sections, Subsections, Exhibits, Schedules
and Appendices of this Agreement unless otherwise indicated. All Exhibits,
Schedules and Appendices attached to this Agreement are incorporated herein and
made a part hereof.

SECTION 2.   THE LOAN.

     2.1     GENERAL.

             (a) Subject to the terms and conditions hereof, the Lender agrees
     to make a loan (the "LOAN") to the Borrower on the Closing Date in the
     original principal amount of Five Million and 00/100 Dollars
     ($5,000,000.00). The principal amount of the Loan may from time to time be
     advanced as or converted to (i) LIBOR Loans, (ii) Prime Rate Loans or (iii)
     a combination thereof, as determined by the Borrower and notified to the
     Lender in accordance with subsections 2.2 and 2.8.

             (b) On the Closing Date, the Borrower shall execute and deliver to
     the Lender a certain Term Note of the Borrower dated as of the Closing Date
     evidencing the Loan, substantially in the form of EXHIBIT A attached hereto
     (the "NOTE"). The Lender is hereby authorized to record the date, Type and
     amount of the Loan made, the date and amount of each payment or prepayment
     of principal thereof, each continuation thereof, each conversion of all or
     a portion thereof to another Type and, in the case of LIBOR Loans, the
     length of each Interest Period and LIBOR Rate with respect thereto, on the
     schedule (or any continuation of the schedule) annexed to and constituting
     a part of the Note and any such recordation shall, to the extent permitted
     by applicable law, constitute prima facie evidence of the accuracy of the
     information so recorded (absent manifest error); PROVIDED, HOWEVER, that
     the failure to make any such recordation (or any error

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     therein) shall not affect the obligation of the Borrower to repay (with
     applicable interest) the Loan.

     2.2     PROCEDURE FOR BORROWING. If all or any part of the Loan is to be
initially LIBOR Loans, the Borrower shall give the Lender irrevocable notice
thereof (which notice must be received by the Lender prior to 3:00 p.m., Boston,
Massachusetts time, three (3) Business Days prior to the Closing Date),
specifying (a) the amount of the Loan which is initially to be LIBOR Loans and
(b) the respective amounts thereof and the lengths of the initial Interest
Periods therefor. To the extent that the Borrower does not deliver a notice
pursuant to the immediately preceding sentence, the Loan shall initially be
Prime Rate Loans. On the Closing Date, the Lender will credit the account of the
Borrower on the books of the Lender with the amount so borrowed.

     2.3     USE OF PROCEEDS. The proceeds of the Loan shall be used by the
Borrower to refinance the existing term loan made by Fleet National Bank to the
Borrower, finance capital expenditures and for general corporate purposes.

     2.4     INTEREST RATES.

             (a) Each portion of the unpaid principal balance of the Loan which
     is a LIBOR Loan shall bear interest, for each Interest Period applicable
     thereto, at a rate per annum equal to the LIBOR Rate plus 2.75%.

             (b) Each portion of the unpaid principal balance of the Loan which
     is a Prime Rate Loan shall bear interest at a rate per annum equal to the
     Prime Rate plus 0.5%.

     2.5     PAYMENT OF TERM LOAN. The Borrower hereby unconditionally promises
to pay to the order of the Lender the principal amount of the Loan in quarterly
installments of Five Hundred Thousand and 00/100 Dollars ($500,000.00) each,
commencing on June 30, 2003, and continuing on the last day of each consecutive
quarter thereafter with a final payment of One Million and 00/100 Dollars
($1,000,000) on the Maturity Date. The Borrower hereby further agrees to pay
interest on the unpaid principal balance of the Loan, in arrears, on each
Interest Payment Date; provided, however, any such interest accruing at the Late
Rate shall be due and payable on demand. On the Maturity Date (or such earlier
date on which the Loan becomes due and payable pursuant to subsection 7.1), the
entire remaining outstanding balance of the Loan (including, without limitation,
all unpaid principal, all accrued but unpaid interest and all unpaid fees,
charges, costs and expenses) shall be immediately due and payable in full.

     2.6     METHOD OF PAYMENT. All payments (including prepayments) to be made
by the Borrower hereunder, whether on account of principal, interest, fees or
otherwise, shall be made without set off or counterclaim and shall be made prior
to 3:00 p.m., Boston, Massachusetts time, on the due date thereof to the Lender,
at the Lender's office specified in subsection 9.4 (or such other place as the
Lender may specify in writing from time to time), in Dollars and in immediately
available funds. Payments received by the Lender after such time shall be deemed
to have been received on the next Business Day. If any payment hereunder (other
than payments on LIBOR Loans) becomes due and payable on a day other than a
Business Day, such payment shall be extended to the next succeeding Business
Day, and, with respect to payments of

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principal, interest thereon shall be payable at the then applicable rate during
such extension. If any payment on a LIBOR Loan becomes due and payable on a day
other than a Business Day, the maturity thereof shall be extended to the next
succeeding Business Day (and, with respect to payments of principal, interest
shall be payable thereon at the then applicable rate during such extension)
unless the result of such extension would be to extend such payment into another
calendar month, in which event such payment shall be made on the immediately
preceding Business Day. The Borrower hereby authorizes the Lender to charge or
debit any deposit account of the Borrower with the Lender to effect (i) any
payment of principal or interest due hereunder and (ii) any other payment
invoiced by the Lender to the Borrower and not paid by the Borrower when due.

     2.7     PREPAYMENTS.

             (a) OPTIONAL PREPAYMENT. The Borrower may at any time and from time
     to time prepay the Loan, in whole or in part, without premium or penalty
     (other than as provided in subsection 2.16), upon irrevocable notice to the
     Lender prior to 3:00 p.m., Boston, Massachusetts time, one (1) Business Day
     prior to such prepayment, specifying the date and amount of prepayment and
     whether the prepayment is of LIBOR Loans, Prime Rate Loans or a combination
     thereof, and, if of a combination thereof, the amount allocable to each;
     PROVIDED, HOWEVER, notwithstanding the foregoing to the contrary, no such
     notice shall be required in connection with the prepayment of any Prime
     Rate Loan. If any such notice is given, the amount specified in such notice
     shall be due and payable on the date specified therein, together with any
     amounts payable pursuant to subsection 2.16. Amounts prepaid on account of
     the Loan may not be reborrowed. Partial prepayments shall be in the
     aggregate principal amount of One Hundred Thousand and 00/100 Dollars
     ($100,000.00) or a whole multiple of Fifty Thousand and 00/100 Dollars
     ($50,000.00) in excess thereof. No partial prepayment shall postpone or
     extend the date on which any principal or interest is otherwise due under
     any Loan hereunder.

             (b) APPLICATION OF PREPAYMENTS. All amounts received for the
     prepayment of Loans shall be applied to the Obligations as follows, so long
     as no Event of Default has occurred and is continuing: first, to any fees,
     charges, costs and expenses then owed by the Borrower to the Lender and
     second, to the unpaid principal balance of the Loan in inverse order of
     maturity. The application of any prepayment pursuant to this subsection
     shall be made first to the Base Rate Loans and second to the LIBOR Loans.
     Upon the occurrence and during the continuation of any Event of Default,
     all amounts received for the prepayment of Loans shall be applied to the
     Obligations in such manner as the Lender may reasonably determine.

             (c) MANDATORY PREPAYMENTS. The Borrower shall prepay the Loan in
     whole or in part in inverse order of maturity from any Net Proceeds within
     thirty (30) days after such Net Proceeds are received by the Borrower,
     provided that such Net Proceeds are greater than or equal to One Hundred
     Thousand and 00/100 Dollars ($100,000).

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     2.8     CONVERSION AND CONTINUATION OPTIONS.

             (a) The Borrower may elect from time to time to convert LIBOR Loans
     to Prime Rate Loans by giving the Lender at least one (1) Business Day's
     prior irrevocable notice of such election, provided that any such
     conversion of LIBOR Loans may only be made on the last day of an Interest
     Period with respect thereto (or on any other day if on the date of such
     conversion the Borrower pays to the Lender accrued interest on such LIBOR
     Loans to the date of such conversion together with all amounts payable
     under subsection 2.16). The Borrower may elect from time to time to convert
     Prime Rate Loans to LIBOR Loans by giving the Lender at least three (3)
     Business Days' prior irrevocable notice of such election. Any such notice
     of conversion to LIBOR Loans shall specify the length of the initial
     Interest Period or Interest Periods therefor. All or any part of
     outstanding LIBOR Loans and Prime Rate Loans may be converted as provided
     herein; PROVIDED, HOWEVER, that (i) no Loan may be converted into a LIBOR
     Loan when any Event of Default has occurred and is continuing and the
     Lender has determined that such a conversion is not appropriate and (ii) no
     Loan may be converted into a LIBOR Loan after the date that is one month
     prior to the Maturity Date.

             (b) Any LIBOR Loans may be continued as such upon the expiration of
     the then current Interest Period with respect thereto by the Borrower
     giving notice to the Lender, in accordance with the applicable provisions
     of the term "Interest Period" set forth in APPENDIX A, of the length of the
     next Interest Period to be applicable to such Loans; PROVIDED, HOWEVER,
     that no LIBOR Loan may be continued as such (i) when any Event of Default
     has occurred and is continuing and the Lender has determined that such a
     continuation is not appropriate or (ii) after the date that is one month
     prior to the maturity of the Loan; and PROVIDED FURTHER that if the
     Borrower shall fail to give such notice or if such continuation is not
     permitted, such Loans shall be automatically converted to Base Rate Loans
     on the last day of such then expiring Interest Period.

     2.9     MINIMUM AMOUNTS AND MAXIMUM NUMBER OF LIBOR LOANS. All borrowings,
conversions and continuations of LIBOR Loans and Prime Rate Loans hereunder and
all selections of Interest Periods hereunder shall be in such amounts and be
made pursuant to such elections so that, after giving effect thereto, the
aggregate principal amount of the Loan comprising each LIBOR Loan shall be equal
to Five Hundred Thousand and 00/100 Dollars ($500,000.00) or a whole multiple of
One Hundred Thousand and 00/100 Dollars ($100,000.00) in excess thereof. In no
event shall there be more than Five (5) LIBOR Loans outstanding at any time.

     2.10    LATE RATE; LATE CHARGE.

             (a) Notwithstanding any provision contained in this Agreement or
     any other Financing Document to the contrary, if all or a portion of (i)
     the principal amount of the Loan, (ii) any interest payable thereon or
     (iii) any fee or other amount payable hereunder shall not be paid when due
     (whether at the stated maturity, by acceleration or otherwise, but taking
     into account any applicable grace period under subsection 7.1(a)), such
     overdue amount shall bear interest at a rate per annum (hereinafter
     referred to as the "LATE RATE") which is (x) in the case of overdue
     principal, the rate that would otherwise

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     be applicable to the Loan under this Agreement, plus Two Percent (2.0%) or
     (y) in the case of overdue interest, fees or other amounts due and payable
     hereunder, the rate that would otherwise be applicable under this Agreement
     to that portion of the unpaid principal amount of the Loan that is a Prime
     Rate Loan, plus Two Percent (2.0%), in each case from the date of such
     non-payment until such amount is paid in full (after as well as before
     judgment).

             (b) The Borrower agrees to pay, on demand and in addition to all
     other amounts payable under this Agreement and the other Financing
     Documents, a late charge on any payment that is more than ten (10) calendar
     days late, which late charge shall be equal to Thirty Five and 00/100
     Dollars ($35.00). The assessment or collection of late charges is not
     intended and shall not be construed to permit payment of any amount payable
     under this Agreement or any of the other Financing Documents beyond the
     applicable due date thereof. The time period which is allowed before the
     assessment of late charges is not intended and shall not be construed as a
     grace or cure period with respect to payment or performance of any
     obligation under this Agreement or any of the other Financing Documents.

             (c) Notwithstanding any provision contained in this Agreement or
     any other Financing Document to the contrary, in no event shall the amount
     paid or agreed to be paid by the Borrower (or any other Person) as interest
     or as a premium on the Loan or any other Obligations exceed the highest
     lawful rate permissible under any law applicable thereto.

     2.11    COMPUTATION OF INTEREST AND FEES.

             (a) Interest and all fees payable hereunder shall be computed daily
     on the basis of a year of 360 days and paid for the number of actual days
     for which due. The Lender shall as soon as practicable, notify the Borrower
     of each determination of a LIBOR Rate. Any change in the interest rate on a
     Loan resulting from a change in the Prime Rate or the LIBOR Reserve
     Requirement shall become effective as of the opening of business on the day
     on which such change becomes effective. The Lender shall, as soon as
     practicable, notify the Borrower of the effective date and the amount of
     each such change in interest rate.

             (b) Each determination of an interest rate by the Lender pursuant
     to any provision of this Agreement shall be conclusive and binding on the
     Borrower in the absence of manifest error. The Lender shall, at the request
     of the Borrower, deliver to the Borrower a statement showing the quotations
     used by the Lender in determining any interest rate hereunder.

     2.12    INABILITY TO DETERMINE INTEREST RATE. If prior to the first day of
any Interest Period:

             (a) the Lender shall have determined (which determination shall be
     conclusive and binding upon the Borrower) that, by reason of circumstances
     affecting the relevant

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     market, adequate and reasonable means do not exist for ascertaining the
     LIBOR Rate for such Interest Period, or

             (b) the Lender shall have determined (which determination shall be
     conclusive and binding upon the Borrower) that the LIBOR Rate determined or
     to be determined for such Interest Period will not adequately and fairly
     reflect the cost to the Lender of making or maintaining its affected LIBOR
     Loans during such Interest Period,

the Lender shall give telecopy or telephonic notice thereof to the Borrower as
soon as practicable thereafter. If such notice is given, (x) any LIBOR Loans
requested to be made on the first day of such Interest Period shall be made as
Prime Rate Loans, (y) any Prime Rate Loans that were to have been converted on
the first day of such Interest Period to LIBOR Loans shall be converted to or
continued as Prime Rate Loans and (z) any outstanding LIBOR Loans shall be
converted, on the first day of such Interest Period, to Prime Rate Loans. Until
such notice has been withdrawn by the Lender, no further LIBOR Loans shall be
made or continued as such, nor shall the Borrower have the right to convert
Prime Rate Loans to LIBOR Loans.

     2.13    ILLEGALITY. Notwithstanding any other provision herein, if the
adoption of or any change in any Requirement of Law or in the interpretation or
application thereof shall make it unlawful for the Lender to make or maintain
LIBOR Loans as contemplated by this Agreement, (a) the commitment of the Lender
hereunder to make LIBOR Loans, continue LIBOR Loans as such and convert Prime
Rate Loans to LIBOR Loans shall forthwith be suspended until such time as it
shall no longer be unlawful for the Lender to make or maintain LIBOR Loans as
contemplated by this Agreement and (b) the portion of the Lender's Loan then
outstanding as LIBOR Loans, if any, shall be converted automatically to Prime
Rate Loans on the respective last days of the then current Interest Periods with
respect to such LIBOR Loans or within such earlier period as required by law. If
any such conversion of a LIBOR Loan occurs on a day which is not the last day of
the then current Interest Period with respect thereto, the Borrower shall pay to
the Lender such amounts, if any, as may be required pursuant to subsection 2.16.

     2.14    INCREASED COSTS; CHANGED IN REQUIREMENTS OF LAW.

             (a) If the adoption of or any change in any Requirement of Law or
     in the interpretation or application thereof or compliance by the Lender
     with any request or directive (whether or not having the force of law) from
     any central bank or other Governmental Authority, in each case made
     subsequent to the date hereof:

               (i)     shall subject the Lender to any tax of any kind
             whatsoever with respect to this Agreement, the Note or any LIBOR
             Loan made by it, or change the basis of taxation of payments to the
             Lender in respect thereof (except for Non-Excluded Taxes, changes
             in the rate of tax on the overall net income of the Lender and
             taxes imposed as a result of any future, present or former
             connection between the Lender and the jurisdiction of the
             Governmental Authority imposing such tax or any political
             subdivision or taxing authority thereof or therein (other than any
             such connection arising solely from the Lender having executed,
             delivered or performed its obligations or received a payment under,
             or enforced, this Agreement or the Note));

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               (ii)    shall impose, modify or hold applicable any reserve,
             special deposit, compulsory loan or similar requirement against
             assets held by, deposits or other liabilities in or for the account
             of, advances, loans or other extensions of credit by, or any other
             acquisition of funds by, any office of the Lender which is not
             otherwise included in the determination of the LIBOR Rate
             hereunder; or

               (iii)   shall impose on the Lender any other condition; and the
             result of any of the foregoing is to increase the cost to the
             Lender, by an amount which the Lender reasonably deems to be
             material, of making, converting into, continuing or maintaining
             LIBOR Loans or to reduce any amount receivable hereunder in respect
             thereof, then, in any such case, the Borrower shall, within ten
             (10) Business Days after receipt by the Borrower of the Lender's
             written demand, pay the Lender such additional amount or amounts as
             will compensate the Lender for such increased cost or reduced
             amount receivable. If the Lender has demanded such compensation
             under this subsection with respect to any LIBOR Loan, the Borrower
             shall have the option to convert immediately such LIBOR Loan into a
             Prime Rate Loan until the circumstances giving rise to such demand
             for compensation no longer apply; PROVIDED, HOWEVER, that (i) no
             such conversion shall affect the Borrower's obligation to pay
             compensation as provided herein which is due with respect to the
             period prior to such conversion and (ii) on the date of such
             conversion the Borrower shall pay to the Lender accrued interest on
             such LIBOR Loan to the date of conversion, together with any
             amounts payable pursuant to subsection 2.16.

             (b) If the Lender shall have determined that the adoption of or any
     change in any Requirement of Law regarding capital adequacy or in the
     interpretation or application thereof or compliance by the Lender or any
     corporation controlling the Lender with any request or directive regarding
     capital adequacy from any Governmental Authority, in each case made
     subsequent to the date hereof, shall have the effect of reducing the rate
     of return on the Lender's or such corporation's capital as a consequence of
     its obligations hereunderto a level below that which the Lender or such
     corporation could have achieved but for such adoption, change or compliance
     (taking into consideration the Lender's or such corporation's policies with
     respect to capital adequacy) by an amount reasonably deemed by the Lender
     to be material, then from time to time, within ten (10) Business Days after
     receipt by the Borrower of the Lender's written demand, the Borrower shall
     pay to the Lender such additional amount or amounts as will compensate the
     Lender for such reduction.

     2.15    TAXES.

             (a) All payments made by the Borrower under this Agreement and the
     Note shall be made free and clear of, and without deduction or withholding
     for or on account of, any present or future income, stamp or other taxes,
     levies, imposts, duties, charges, fees, deductions or withholdings, now or
     hereafter imposed, levied, collected, withheld or assessed by any
     Governmental Authority, excluding net income taxes and franchise taxes
     (imposed in lieu of net income taxes) imposed on the Lender as a result of
     any future, present or former connection between the Lender and the
     jurisdiction of the

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     Governmental Authority imposing such tax or any political subdivision or
     taxing authority thereof or therein (other than any such connection arising
     solely from the Lender having executed, delivered or performed its
     obligations or received a payment under, or enforced, this Agreement or the
     Note or any other Financing Document). If any such non-excluded taxes,
     levies, imposts, duties, charges, fees deductions or withholdings
     ("NON-EXCLUDED TAXES") are required to be withheld from any amounts payable
     to the Lender hereunder or under the Note, the amounts so payable to the
     Lender shall be increased ("INCREASED AMOUNTS") to the extent necessary to
     yield to the Lender (after payment of all Non-Excluded Taxes) interest or
     any such other amounts payable hereunder at the rates or in the amounts
     specified in this Agreement. Whenever any Non-Excluded Taxes are payable by
     the Borrower, the Borrower shall promptly send to the Lender, a certified
     copy of an original official receipt received by the Borrower showing
     payment thereof or other evidence of remittance of Non-Excluded Taxes
     reasonably acceptable to the Lender. If the Borrower fails to pay any
     Non-Excluded Taxes when due to the appropriate taxing authority or fails to
     remit to the Lender the required receipts or other reasonably acceptable
     evidence, the Borrower shall indemnify the Lender for any incremental
     taxes, interest or penalties that may become payable by the Lender as a
     result of any such failure. The Borrower will indemnify the Lender for the
     amount of Non-Excluded Taxes paid by the Lender, and any liability
     (including penalties, interest and expenses) arising therefrom or with
     respect thereto. The agreements in this subsection 2.15 shall survive the
     termination of this Agreement and the payment of the Loan and all other
     amounts payable hereunder.

             (b) If the Lender shall become aware that it is entitled to claim a
     refund from a Governmental Authority in respect of Non-Excluded Taxes as to
     which it has been indemnified by the Borrower, or with respect to which the
     Borrower has paid Increased Amounts, pursuant to this subsection, it shall
     promptly notify the Borrower of the availability of such refund claim and
     shall make the appropriate claim to such Governmental Authority for such
     refund. If the Lender receives a refund (including pursuant to a claim for
     refund made pursuant to the preceding sentence) in respect of any
     Non-Excluded Tax as to which it has been indemnified by the Borrower, or
     with respect to which the Borrower has paid Increased Amounts pursuant to
     this subsection, it shall within thirty (30) days from the date of such
     receipt pay over such refund to the Borrower, net of all out-of-pocket
     third-party expenses of the Lender.

     2.16    BREAKAGE - INDEMNITY. The Borrower agrees to indemnify the Lender
and to hold the Lender harmless from any loss or expense which the Lender may
sustain or incur as a consequence of (a) default by the Borrower in making a
borrowing of, conversion into or continuation of LIBOR Loans after the Borrower
has given a notice requesting the same in accordance with the provisions of this
Agreement, (b) default by the Borrower in making any prepayment after the
Borrower has given a notice thereof in accordance with the provisions of this
Agreement or (c) the making of a prepayment of LIBOR Loans or converting any
LIBOR Loans to Prime Rate Loans on a day which is not the last day of an
Interest Period with respect thereto. Such indemnification may include an amount
equal to the excess, if any, of (i) the amount of interest which would have
accrued on the amount so prepaid, or not so borrowed, converted or continued,
for the period from the date of such prepayment or of such failure to

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borrow, convert or continue to the last day of such Interest Period (or, in the
case of a failure to borrow, convert or continue, the last day of the Interest
Period that would have commenced on the date of such failure) in each case at
the applicable rate of interest for such LIBOR Loans provided for herein over
(ii) the amount of interest (as reasonably determined by the Lender) which would
have accrued to the Lender on such amount by placing such amount on deposit for
a comparable period with leading banks in the London interbank market. This
covenant shall survive the termination of this Agreement and the payment of the
Loan and all other amounts payable hereunder.

SECTION 3.   CONDITIONS PRECEDENT.

     The effectiveness of this Agreement and the agreement of the Lender to make
the initial Extension of Credit requested to be made by it is subject to the
satisfaction, immediately prior to or concurrently with the making of such
Extension of Credit on the Closing Date, of the following conditions precedent:

     3.1     FINANCING DOCUMENTS. The Lender shall have received the following
Financing Documents, each as duly executed by the parties thereto, with their
signatures properly witnessed and notarized thereon where indicated: (i) this
Agreement; (ii) the Note conforming to the requirements hereof; (iii) the
Subsidiary Guaranty; and (iv) the Security Agreements.

     3.2     ACTIONS TO PERFECT LIENS. The Lender shall have received evidence
in form and substance reasonably satisfactory to it that all filings, recordings
and registrations, including, without limitation, the filing of duly executed
financing statements on form UCC-1, necessary or, in the opinion of the Lender,
desirable to perfect the Liens created by the Security Documents shall have been
completed (or, to the extent that any such filings, recordings, registrations
and other actions shall not have been completed, arrangements satisfactory to
the Lender for the completion thereof shall have been made).

     3.3     PLEDGED STOCK; STOCK POWERS. The Lender shall have received the
original certificates representing the shares of Capital Stock pledged pursuant
to the Security Documents, together with an undated stock power for each such
certificate executed in blank by a duly authorized officer of the pledgor.

     3.4     LIEN SEARCHES. The Lender shall have received the results of a
recent search by a Person reasonably satisfactory to the Lender, of the UCC,
judgment and tax lien filings which may have been filed with respect to personal
property of the Borrower and each of its Subsidiaries in the jurisdictions set
forth in APPENDIX B, and the results of such search shall be satisfactory to the
Lender.

     3.5     PAYOFF LETTER. The Lender shall have received a payoff letter from
Fleet National Bank, setting forth in reasonable detail the total amount of
Indebtedness owed by the Borrower to Fleet National Bank as of the Closing Date,
together with appropriate wire instructions attached thereto.

     3.6     UCC-3 TERMINATION STATEMENTS. The Lender shall have received UCC-3
termination statements and any other instrument necessary to terminate the Liens
granted by the Borrower to any Person (other than Permitted Liens) (or, to the
extent that any such UCC-3 termination statements or any other instrument shall
not have been obtained and filed,

                                       -9-
<Page>

arrangements satisfactory to the Lender for the obtaining and filing thereof
shall have been made).

     3.7     CORPORATE PROCEEDINGS OF THE BORROWER. The Lender shall have
received a copy of the resolutions, in form and substance satisfactory to the
Lender, of the Board of Directors of the Borrower authorizing (i) the execution,
delivery and performance of this Agreement and the other Financing Documents to
which it is a party, (ii) the Extensions of Credit contemplated hereunder and
(iii) the granting by it of the Liens created pursuant to the Security Documents
to which the Borrower is a party, all as certified by the Secretary or an
Assistant Secretary of the Borrower as of the Closing Date, which certificate
shall be in form and substance reasonably satisfactory to the Lender and shall
state that the resolutions thereby certified have not been amended, modified,
revoked or rescinded.

     3.8     BORROWER INCUMBENCY CERTIFICATE. The Lender shall have received a
certificate of the Borrower, dated as of the Closing Date, as to the incumbency
and signature of the officers of the Borrower executing any Financing Document
reasonably satisfactory in form and substance to the Lender, executed by the
President or any Vice President and the Secretary or an Assistant Secretary of
the Borrower.

     3.9     CORPORATE PROCEEDINGS OF SUBSIDIARIES. The Lender shall have
received a copy of the resolutions, in form and substance reasonably
satisfactory to the Lender, of the Board of Directors of each Subsidiary of the
Borrower authorizing (i) the execution, delivery and performance of the
Financing Documents to which it is a party and (ii) the granting by it of the
Liens created pursuant to the Security Documents to which it is a party, all as
certified by the Secretary or an Assistant Secretary of each such Subsidiary as
of the Closing Date, which certificate shall be in form and substance reasonably
satisfactory to the Lender and shall state that the resolutions thereby
certified have not been amended, modified, revoked or rescinded.

     3.10    SUBSIDIARY INCUMBENCY CERTIFICATES. The Lender shall have received
a certificate of each Subsidiary of the Borrower, dated as of the Closing Date,
as to the incumbency and signature of the officers of each such Subsidiary
executing any Financing Document, reasonably satisfactory in form and substance
to the Lender, executed by the President or any Vice President and the Secretary
or an Assistant Secretary of each such Subsidiary.

     3.11    CORPORATE DOCUMENTS. The Lender shall have received true and
complete copies of the Certificate of Incorporation and By-Laws of each of the
Borrower and its Subsidiaries, as certified as of the Closing Date as complete
and correct copies thereof by the Secretary or an Assistant Secretary of the
Borrower or such Subsidiary, whichever is applicable.

     3.12    LEGAL EXISTENCE, GOOD STANDING, TAX GOOD STANDING AND FOREIGN
QUALIFICATION CERTIFICATES. The Lender shall have received certificates of legal
existence, good standing, tax good standing and foreign qualification for each
of the Borrower and its Subsidiaries, all of recent date issued by the
appropriate Governmental Authorities.

     3.13    INSURANCE. The Lender shall have received evidence in form and
substance satisfactory to it that all of the requirements of subsection 5.5 and
those Sections of the Security Documents requiring the maintenance of insurance
shall have been satisfied.

                                      -10-
<Page>

     3.14    COMPLIANCE CERTIFICATE. The Lender shall have received a
certificate, dated as of the Closing Date, in the form of APPENDIX C hereto,
evidencing compliance with all terms and conditions of this Agreement as of the
Closing Date.

     3.15    LEGAL OPINION. The Lender shall have received an executed legal
opinion of Bingham McCutchen, counsel to the Borrower, covering such matters
related to the transactions contemplated by this Agreement and the other
Financing Documents as the Lender may reasonably request. Such legal opinion
shall be in a form and substance reasonably acceptable to the Lender and its
counsel.

     3.16    FEES AND EXPENSES The Lender shall have received reimbursement or
payment of (1) all reasonable and documented legal fees incurred by the Lender
in connection with the transactions contemplated herein, and (2) all other
reasonable and documented out-of-pocket costs and expenses incurred by the
Lender in connection with the transactions contemplated herein.

SECTION 4.   REPRESENTATIONS AND WARRANTIES.

     To induce the Lender to enter into this Agreement and to make the Loan, the
Borrower hereby represents and warrants to the Lender that, except as otherwise
described in the MASTER DISCLOSURE SCHEDULE attached hereto:

     4.1     FINANCIAL CONDITION. The Borrower has furnished to the Lender the
financial statements of the Borrower for the years ended December 31, 1999,
December 31, 2000, December 31, 2001 and for the nine (9) months ended September
30, 2002 (collectively, the "INITIAL FINANCIAL STATEMENTS"). The Initial
Financial Statements were prepared in accordance with GAAP, consistently
maintained and applied throughout the periods covered thereby (except as may be
noted therein) and fairly present the financial condition of the Borrower on the
respective dates thereof and the results of the Borrower's operations for the
respective periods covered thereby.

     4.2     NO CHANGE. Except as set forth on the MASTER DISCLOSURE SCHEDULE,
since September 30, 2002, (a) there has been no development or event which has
had or could reasonably be expected to have a Material Adverse Effect, (b) there
has been no sale, transfer or other disposition by the Borrower of any material
part of its business or property and no purchase or other acquisition of any
business or property (including any Capital Stock of the Borrower) material in
relation to the financial condition of the Borrower on September 30, 2002, and
(c) no dividends or other distributions have been declared, paid or made upon
the Capital Stock of the Borrower (other than those dividends and distributions
permitted pursuant to subsection 6.9(a)) nor has any of the Capital Stock of the
Borrower been redeemed, retired, purchased or otherwise acquired for value by
the Borrower.

     4.3     EXISTENCE; COMPLIANCE WITH LAW. Each of the Borrower and its
Subsidiaries (a) is duly organized, validly existing and in good standing under
the laws of the jurisdiction of its organization, (b) has the power and
authority to own and operate its property, to lease the property it operates as
lessee and to conduct the business in which it is currently engaged, (c) is duly
qualified or licensed to do business as a foreign company and in good standing
under the

                                      -11-
<Page>

laws of each jurisdiction where its ownership, lease or operation of property or
the conduct of its business requires such qualification except where the failure
to be so qualified and/or in good standing, in the aggregate could not
reasonably be expected to have a Material Adverse Effect and (d) is in
compliance with all Requirements of Law and applicable Charter Documents except
to the extent that the failure to comply therewith could not, in the aggregate,
reasonably be expected to have a Material Adverse Effect.

     4.4     POWER; AUTHORIZATION; ENFORCEABLE OBLIGATIONS. Each of the Borrower
and its Subsidiaries has the power and authority, and the legal right, to make,
deliver and perform the Financing Documents to which it is a party and, in the
case of the Borrower, to borrow hereunder. Each of the Borrower and its
Subsidiaries has taken all necessary action to authorize the Loan on the terms
and conditions of this Agreement and the Note and to authorize the execution,
delivery and performance by it of the Financing Documents to which it is a
party. No consent or authorization of, filing with, notice to or other act by or
in respect of, any Governmental Authority or any other Person is required to be
obtained or made by the Borrower or any of its Subsidiaries in connection with
the Loan hereunder or with the execution, delivery or performance by the
Borrower or any of its Subsidiaries or the validity or enforceability with
respect to or against the Borrower or any of its Subsidiaries, as the case may
be, of the Financing Documents to which the Borrower or such Subsidiary, as the
case may be, is a party (other than the filings of Uniform Commercial Code
financing statements in order to perfect the security interest that can be
perfected by such filings). Each of the Financing Documents, when executed and
delivered, will constitute a legal, valid and binding obligation of each of the
Borrower and its Subsidiaries, as the case may be, enforceable against each of
them, as the case may be, to the extent that each of them is a party thereto,
except as such enforceability may be limited by applicable bankruptcy,
insolvency, reorganization, moratorium or similar laws affecting the enforcement
of creditors' rights generally and by general equitable principles (whether
enforcement is sought by proceedings in equity or at law).

     4.5     NO LEGAL BAR. The execution, delivery and performance of the
Financing Documents, the Loan hereunder and the use of the proceeds thereof by
the Borrower will not violate any Requirement of Law, Charter Document or
Contractual Obligation of the Borrower or any of its Subsidiaries which could
reasonably be expected to have a Material Adverse Effect and will not result in,
or require, the creation or imposition of any Lien on any of its or their
respective properties or revenues pursuant to any such Requirement of Law,
Charter Document or Contractual Obligation other than as contemplated in or
permitted by the Financing Documents.

     4.6     NO MATERIAL LITIGATION. No litigation, investigation or proceeding
of or before any arbitrator or Governmental Authority is pending or, to the
knowledge of the Borrower, threatened by or against the Borrower or any of its
Subsidiaries or against any of their respective properties or revenues which has
a reasonable possibility of an adverse determination, and if adversely
determined, could reasonably be expected to have a Material Adverse Effect.

     4.7     NO DEFAULT. Neither the Borrower nor any of its Subsidiaries is in
default under or with respect to any of its Contractual Obligations in any
respect which could reasonably be expected to have a Material Adverse Effect. No
Default or Event of Default has occurred and is continuing hereunder.

                                      -12-
<Page>

     4.8     OWNERSHIP OF PROPERTY; LIENS. Each of the Borrower and its
Subsidiaries has good record and marketable title in fee simple to, or a valid
leasehold interest in, all of its real property except for such matters as do
not materially adversely affect the use of the property in the conduct of the
business as currently conducted, and good title to, or a valid leasehold
interest in, all its other material property, and none of such property is
subject to any Lien (other than Permitted Liens). Set forth on the MASTER
DISCLOSURE SCHEDULE is a true and complete list of all of real property owned or
leased by the Borrower and its Subsidiaries as of the Closing Date and all Liens
granted by the Borrower and its Subsidiaries in respect of any real property
owned or leased by it as of the Closing Date.

     4.9     INTELLECTUAL PROPERTY. (a) Each of the Borrower and its
Subsidiaries owns, or is licensed to use, all material patents, trademarks
(registered or unregistered), trade names, service marks, assumed names and
copyrights (such items, together with all applications therefor and all other
material intellectual property and proprietary rights, whether or not subject to
statutory registration or protection, of the Borrower and any of its
Subsidiaries that are used in or necessary for the conduct of the business of
the Borrower and its Subsidiaries being collectively referred to herein as the
"INTELLECTUAL PROPERTY") necessary for the conduct of its business except for
those the failure to own or license which could not reasonably be expected to
have a Material Adverse Effect and (b) no claim of which the Borrower has been
given notice has been asserted and is pending by any Person challenging or
questioning the use of any such Intellectual Property or the validity or
effectiveness of any such Intellectual Property, nor does the Borrower know of
any valid basis for any such claim, except for such claims that, in the
aggregate, could not reasonably be expected to have a Material Adverse Effect.

     4.10    TAXES. The Borrower and its Subsidiaries have filed or caused to be
filed all tax returns which, to the knowledge of the Borrower and its
Subsidiaries, are required to be filed and have paid all taxes shown to be due
and payable on said returns or on any assessments made against it or any of its
property in respect of such periods and all other material taxes imposed on it
or any of its property by any Governmental Authority (other than any taxes the
amount or validity of which are being contested in good faith by appropriate
proceedings and with respect to which reserves in conformity with GAAP have been
provided on the books of the Borrower and its Subsidiaries and other than any
taxes which in the aggregate would not have a Material Adverse Effect) in
respect of such periods.

     4.11    FEDERAL REGULATIONS. No part of the proceeds of any Loan will be
used for "purchasing" or "carrying" any "margin stock" within the respective
meanings of each of the quoted terms under Regulations G, U or X of the Board of
Governors of the Federal Reserve System as now and from time to time hereafter
in effect. If requested by the Lender, the Borrower will furnish to the Lender a
statement to the foregoing effect in conformity with the requirements of FR Form
U-1 referred to in said Regulation U.

     4.12    ERISA. The Borrower and each Commonly Controlled Entity is in
compliance in all material respects with ERISA and the provisions of the Tax
Code applicable to any Plans. Neither a Reportable Event nor an "accumulated
funding deficiency" (within the meaning of Section 412 of the Tax Code or
Section 302 of ERISA) has occurred with respect to any Plan. Neither the
Borrower nor any Commonly Controlled Entity has incurred any liability to the
PBGC over and above premiums which are required by law and which would
constitute a

                                      -13-
<Page>

Material Adverse Effect. Neither the Borrower nor any Commonly Controlled Entity
has terminated any Plan in a manner which could result in the imposition of a
Lien on the property of any Borrower or its Subsidiaries.

     4.13    INVESTMENT COMPANY ACT; OTHER REGULATIONS. Neither the Borrower nor
any of its Subsidiaries is an "investment company" within the meaning of the
Investment Company Act of 1940, as amended. The Borrower is not subject to
regulation under any Federal or State statute or regulation which limits its
ability to incur Indebtedness as contemplated herein.

     4.14    SUBSIDIARIES. Set forth on the MASTER DISCLOSURE SCHEDULE is a true
and complete list of all of the Subsidiaries of the Borrower as of the Closing
Date.

     4.15    ENVIRONMENTAL MATTERS. Except to the extent that the inaccuracy of
any of the following (or the circumstances giving rise to such inaccuracy),
individually or in the aggregate, could not reasonably be expected to have a
Material Adverse Effect, and to the knowledge any Responsible Officer of the
Borrower:

             (a) The facilities and properties owned, leased or operated by the
     Borrower or any of its Subsidiaries do not contain any Hazardous Materials
     in amounts or concentrations which (i) constitute a violation of, or (ii)
     could give rise to any liability under, any Environmental Law or could
     interfere with the continued operation of the facilities and properties
     owned, leased or operated by the Borrower or any of its Subsidiaries or
     could reasonably be expected to impair the fair saleable value thereof.

             (b) The Borrower and its Subsidiaries, together with all of the
     facilities and properties owned, leased or operated by the Borrower or its
     Subsidiaries, are in compliance, and to the knowledge of the Borrower and
     its Subsidiaries have in the last three years been in compliance with all
     applicable Environmental Laws and applicable Environmental Permits, and the
     Borrower and its Subsidiaries reasonably believe that they will be able to
     comply with all applicable Environmental Laws in the future and renew or
     obtain all Environmental Permits necessary for their operations in the
     future.

             (c) Neither the Borrower nor any of its Subsidiaries has received
     any written notice of violation, alleged violation, non-compliance,
     liability or potential liability regarding environmental matters or
     compliance with Environmental Laws with regard to any of the facilities and
     properties owned, leased or operated by the Borrower or any of its
     Subsidiaries or the business of the Borrower and its Subsidiaries, nor to
     the knowledge of the Borrower or any of its Subsidiaries is such notice
     being threatened.

             (d) Hazardous Materials have not been transported, disposed of,
     emitted, discharged, or otherwise released or threatened to be released,
     nor has their disposal been arranged for, (i) by the Borrower or any of its
     Subsidiaries in violation of, or (ii) in a manner or to a location which
     could reasonably be expected to give rise to liability under, any
     applicable Environmental Law; nor have any Hazardous Materials been
     generated, treated, stored, emitted, discharged or otherwise released or
     threatened to be released or disposed of at, on or under any of the
     facilities and properties owned, leased or operated

                                      -14-
<Page>

     by the Borrower or any of its Subsidiaries in violation of, or in a manner
     that could reasonably be expected to give rise to liability under, any
     applicable Environmental Law.

             (e) No judicial proceeding or governmental or administrative action
     is pending or, to the knowledge of the Borrower or any of its Subsidiaries,
     threatened, under any Environmental Law to which the Borrower or any
     Subsidiary is or to the knowledge of the Borrower or any of its
     Subsidiaries will be named as a party, nor are there any consent decrees or
     other decrees, consent orders, administrative orders or other orders, or
     other administrative or judicial requirements outstanding under any
     Environmental Law with respect to the Borrower or any of its Subsidiaries,
     or any of the facilities and properties owned, leased or operated by the
     Borrower or any of its Subsidiaries.

     4.16    SOLVENCY. As of the Closing Date, after giving effect to the
transactions contemplated to occur on the Closing Date, each of the Borrower and
its Subsidiaries is Solvent.

     4.17    ASSETS OF CUBIST PHARMACEUTICALS CANADA, INC. AND TERRAGEN
DISCOVERY, INC. The Borrower has caused its subsidiaries, Cubist Pharmaceuticals
Canada, Inc. and TerraGen Discovery, Inc. to transfer all of their respective
assets to the Borrower.

SECTION 5.   AFFIRMATIVE COVENANTS.

     The Borrower hereby covenants and agrees that so long as all or any portion
of the Loan is outstanding, that:

     5.1     FINANCIAL STATEMENTS. The Borrower shall furnish to the Lender the
following financial statements:

             (a) As soon as available, but in any event within ninety (90) days
     after the end of each fiscal year of the Borrower, a copy of the audited
     consolidated balance sheet of the Borrower and its consolidated
     Subsidiaries as at the end of such fiscal year and the related audited
     consolidated statements of income and retained earnings and of cash flows
     for such fiscal year, setting forth in each case in comparative form the
     figures for the previous year, reported on without a "going concern" or
     like qualification or exception, or a qualification arising out of the
     scope of the audit, by PriceWaterhouseCoopers LLP or such other independent
     certified public accountants reasonably satisfactory to the Lender (it
     being understood that the requirements of the provisions of this clause (a)
     for any fiscal year may be satisfied by delivery of a copy of the
     Borrower's annual report on Form 10-K for such fiscal year); and

             (b) As soon as available, but in any event not later than
     forty-five (45) days after the end of each of the first 3 quarterly periods
     of each fiscal year of the Borrower, the management-prepared consolidated
     balance sheet of the Borrower and its consolidated Subsidiaries as at the
     end of such quarter and the related management-prepared consolidated
     statements of income and retained earnings and of cash flows of the
     Borrower and its consolidated Subsidiaries for such quarter and the portion
     of the fiscal year through the end of such quarter, setting forth in each
     case in comparative form the figures for the previous year, certified by a
     Responsible Officer of the Borrower as being fairly stated in all material
     respects (subject to normal year-end audit adjustments) (it

                                      -15-
<Page>

     being understood that the requirements of the provisions of this clause (b)
     for any quarterly period may be satisfied by delivery of a copy of the
     Borrower's quarterly report on Form 10-Q for such fiscal year).

             (c) Concurrently with the delivery of the financial statements
     referred to in subsections 5.1(a) and 5.1(b), the Borrower shall furnish to
     the Lender a compliance certificate ("Compliance Certificate"), signed by
     the Chief Financial Officer of the Borrower, certifying that as of the end
     of such quarter the Borrower is in full compliance with the terms and
     conditions of this Agreement.

             (d) As soon as available, but in any event, within forty five (45)
     days after the issuance thereof, the Borrower shall furnish to the Lender
     copies of such other financial statements, proxy materials and reports as
     it shall send or make available to its stockholders, and promptly upon the
     filing thereof, copies of all reports and materials which the Borrower
     files with any governmental commission (including, without limitation the
     SEC), department or agency or with any domestic or foreign stock exchange
     or with the NASDAQ, including without limitation, copies of (i) any
     registration statements, prospectuses and any amendments and supplements
     thereto, and any regular and periodic reports (including, without
     limitation, reports on Form 10-K, Form 10-Q and Form 8-K) filed by the
     Borrower with the SEC or any domestic or foreign stock exchange or with the
     NASDAQ; and (ii) any letter of comment or correspondence with respect to
     filings or compliance matters sent to the Borrower by any such governmental
     commission (including without limitation, the SEC), department or agency or
     any such domestic or foreign stock exchange or the NASDAQ; provided that
     the foregoing provisions shall not apply to reports, materials, letters or
     correspondence (other than those filed with or received from the SEC) filed
     or received by the Borrower in the ordinary course of business or which
     otherwise do not involve matters that could result in a Material Adverse
     Effect.

All such financial statements shall be complete and correct in all material
respects and shall be prepared in reasonable detail and in accordance with GAAP
applied consistently throughout the periods reflected therein and with prior
periods (except as approved by such accountants or Responsible Officer, as the
case may be, and disclosed therein).

     5.2     CERTIFICATES; OTHER INFORMATION. The Borrower shall furnish to the
Lender the following certificates and other information:

             (a) concurrently with the delivery of the financial statements
     referred to in subsection 5.1(a), a certificate of the independent
     certified public accountants reporting on such financial statements stating
     that in making the examination necessary therefor no knowledge was obtained
     of any Default or Event of Default, except as specified in such
     certificate;

             (b) not later than forty-five (45) days after the beginning of each
     fiscal year of the Borrower, a copy of the projections by the Borrower of
     the balance sheet, income statement and cash flow statement of the Borrower
     and its Subsidiaries for such fiscal year, such projections to be
     accompanied by a certificate of a Responsible Officer of the

                                      -16-
<Page>

     Borrower to the effect that such projections have been prepared using
     assumptions believed in good faith by management of the Borrower to be
     reasonable at the time made and that such Responsible Officer has no reason
     to believe that such projections are incorrect or misleading in any
     material respect;

             (c) within fifteen (15) calendar days after the same are sent,
     copies of all other financial statements and reports which the Borrower
     sends to the holders of any class of its debt securities or public equity
     securities and within fifteen (15) calendar days after the same are filed,
     copies of all other financial statements and reports which the Borrower may
     make to, or file with, the SEC or any successor or analogous Governmental
     Authority; and

     The Borrower shall, and shall cause each of its Subsidiaries to, furnish to
the Lender promptly, such additional financial and other information within the
possession of the Borrower or any of its Subsidiaries as the Lender may from
time to time reasonably request.

     5.3     PAYMENT OF OBLIGATIONS. The Borrower shall, and shall cause each of
its Subsidiaries to, pay, discharge or otherwise satisfy at or before maturity
or before they become delinquent, as the case may be, all of the material
obligations of the Borrower or such Subsidiary, whichever is applicable, except
as contemplated by this Agreement or where the amount or validity thereof is
currently being contested in good faith by appropriate proceedings and reserves
in conformity with GAAP with respect thereto have been provided on the books of
the Borrower or any of its Subsidiaries, as the case may be.

     5.4     CONDUCT OF BUSINESS AND MAINTENANCE OF EXISTENCE. The Borrower
shall, and shall cause each of its Subsidiaries to: (a) continue to engage in
business of the same general type as now conducted by Borrower and each of its
Subsidiaries; (b) preserve, renew and keep in full force and effect its
existence and take all reasonable action to maintain in all material respects
all rights, privileges and franchises necessary or desirable in the normal
conduct of its business except as otherwise permitted pursuant to subsection
6.6; and (c) comply in all material respects with all Contractual Obligations
and Requirements of Law, except where (i) any such Contractual Obligation is
being contested in good faith, a bona fide dispute exists with respect to any
such Contractual Obligation or failure to comply therewith could not, in the
aggregate, reasonably be expected to have a Material Adverse Effect, and (ii)
any such Requirement of Law is being contested in good faith and the failure to
comply therewith could not, in the aggregate, reasonably be expected to have a
Material Adverse Effect.

     5.5     MAINTENANCE OF PROPERTY; INSURANCE. The Borrower shall, and shall
cause each of its Subsidiaries to: (a) keep all property material to the conduct
of its business in good working order and condition; (b) maintain insurance with
financially sound and reputable insurance companies on such of its property and
in at least such amounts and against at least such risks as are usually insured
against in the same general area by companies engaged in the same or a similar
business; and (c) furnish to the Lender, upon written request, full information
as to the insurance carried.

     5.6     INSPECTION OF PROPERTY; BOOKS AND RECORDS; DISCUSSIONS. The
Borrower shall, and shall cause each of its Subsidiaries to: (a) keep proper
financial records in conformity with

                                      -17-
<Page>

GAAP and all Requirements of Law; (b) permit representatives of the Lender to
visit and inspect any of its properties and examine and make abstracts from any
of its books and records at any reasonable time, upon reasonable notice, once
per year if no Event of Default has occurred and is continuing and as often as
may reasonably be desired if an Event of Default has occurred and is continuing;
and (c) permit, upon reasonable notice during normal business hours,
representatives of the Lender to discuss the business, operations, properties
and financial and other condition of the Borrower and its Subsidiaries with
executive officers of the Borrower and its Subsidiaries.

     5.7     NOTICES. The Borrower shall, and shall cause each of its
Subsidiaries to, give prompt notice to the Lender of:

             (a) the occurrence of any Default or Event of Default;

             (b) any (i) default or event of default under any Contractual
     Obligation of the Borrower or any of its Subsidiaries or (ii) litigation,
     investigation or proceeding which may exist at any time between the
     Borrower or any of its Subsidiaries and any Governmental Authority, which
     in either case, if not cured, or resolved or if adversely determined, as
     the case may be, could reasonably be expected to have a Material Adverse
     Effect;

             (c) any litigation or proceeding affecting the Borrower or any of
     its Subsidiaries in which the amount involved is not covered by insurance
     or in which injunctive or similar relief is sought which, if adversely
     determined, could reasonably be expected to have a Material Adverse Effect;

             (d) the following events, as soon as possible and in any event
     within thirty (30) days after the Borrower knows or has reason to know
     thereof: (i) the occurrence or expected occurrence of any Reportable Event
     with respect to any Plan, a failure to make any required contribution to a
     Plan, the creation of any Lien in favor of the PBGC or a Plan or (ii) the
     institution of proceedings or the taking of any other action by the PBGC or
     the Borrower or any Commonly Controlled Entity with respect to the
     withdrawal from, or the termination, reorganization or insolvency of, any
     Plan; and

             (e) any development or event which could reasonably be expected to
     have a Material Adverse Effect.

     Each notice pursuant to this subsection shall be accompanied by a statement
of a Responsible Officer setting forth details of the occurrence referred to
therein and stating what action the Borrower proposes to take with respect
thereto.

     5.8     ENVIRONMENTAL LAWS. The Borrower shall, and shall cause each of its
Subsidiaries to:

             (a) comply with, and use reasonable efforts to ensure compliance by
     all tenants and subtenants, if any, with, all applicable Environmental Laws
     and obtain and comply with and maintain, and use reasonable efforts to
     ensure that all tenants and subtenants, if any, obtain and comply with and
     maintain, any and all Environmental Permits required by applicable
     Environmental Laws; and

                                      -18-
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             (b) Conduct and complete all investigations, studies, sampling and
     testing, and all remedial, removal and other actions required under
     Environmental Laws and promptly comply with all lawful orders and
     directives of all Governmental Authorities regarding Environmental Laws,
     except to the extent that the same are being contested in good faith by
     appropriate proceedings and the pendency of such proceedings could not be
     reasonably expected to have a Material Adverse Effect.

     5.9     MAINTENANCE OF LIENS OF THE SECURITY DOCUMENTS. The Borrower shall,
and shall cause each of its Subsidiaries to, promptly, upon the reasonable
request of the Lender, at the sole cost and expense of the Borrower and its
Subsidiaries, execute, acknowledge and deliver, or cause the execution,
acknowledgement and delivery of, and thereafter register, file or record, or
cause to be registered, filed or recorded, in an appropriate governmental
office, any document or instrument supplemental to or confirmatory of the
Security Documents or otherwise reasonably deemed by the Lender necessary or
desirable for the continued validity, perfection and priority of the Liens on
the Collateral covered thereby.

     5.10    PLEDGE OF AFTER ACQUIRED PROPERTY. If at any time following the
Closing Date, the Borrower or any of its Subsidiaries shall acquire property of
any nature whatsoever having a value in excess of One Hundred Fifty Thousand and
00/100 Dollars ($150,000.00) which is intended by the terms of the applicable
Security Document to be, but is not, subject to the Liens created by the
Security Documents, the Borrower shall, or shall cause the relevant Subsidiaries
to, as soon as possible and in no event later than thirty (30) days after the
relevant acquisition date and, to the extent permitted by applicable law, grant
to the Lender a first priority (subject to Permitted Liens) Lien on such
property as collateral security for the Obligations pursuant to documentation
reasonably satisfactory in form and substance to the Lender. The Borrower, at
its sole expense, shall execute, acknowledge and deliver, or cause the
execution, acknowledgement and delivery of, and thereafter register, file or
record in an appropriate governmental office, any document or instrument
(including legal opinions, title insurance, consents and corporate documents)
and take all such actions reasonably deemed by the Lender to be necessary or
desirable to ensure the creation, priority and perfection of such Lien.

     5.11    NEW SUBSIDIARIES. The Borrower shall cause, at its sole cost and
expense, each new Domestic Subsidiary of the Borrower created or acquired on or
after the date hereof, promptly upon such creation or acquisition, to execute
and deliver to the Lender the following agreements and documents, which
agreements and documents shall be in form and substance reasonably satisfactory
to the Lender: (a) an instrument (it being acknowledged and agreed that an
instrument in the form attached as Exhibit A to the Subsidiary Guaranty shall
satisfy this requirement) pursuant to which such new Domestic Subsidiary shall
be become a party to the Subsidiary Guaranty as a guarantor thereunder; (b) a
security agreement (it being acknowledged and agreed that such security
agreement shall be substantially in the same form as the Subsidiary Security
Agreement-All Assets), pursuant to which, such Domestic Subsidiary shall grant
to the Lender a first priority perfected security interest in all of its assets
in order to secure the full and prompt payment and performance of the
Obligations; (c) any and all UCC financing statements which the Lender deems
necessary and appropriate in order to perfect its first priority perfected
security interests in all of the assets of such Domestic Subsidiary; and (d)
such other agreements, documents, financing statements, instruments, opinions
and certificates and completion of such other matters, as the Lender may
reasonably deem necessary or appropriate.

                                      -19-
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     Promptly upon the creation or acquisition of any Subsidiary created or
acquired after the date hereof, the Borrower shall cause, at its sole cost and
expense, all of the issued and outstanding shares of Capital Stock, membership
interests and other equity interests of each such Domestic Subsidiary and 65% of
Capital Stock, membership interests and other equity interests of each such
Foreign Subsidiary, as the case may be, to be pledged to the Lender so that the
Lender has a first priority perfected security interest in all such shares,
membership interests and other equity interests. Such pledge shall be pursuant
to a pledge agreement in a form and substance reasonably satisfactory to the
Lender.

     5.12    DEPOSITORY ACCOUNTS. The Borrower shall maintain the Lender as its
primary bank for all depository accounts.

     5.13    INVESTMENTS.

             (a) The Borrower and its Subsidiaries, on a consolidated basis,
     shall maintain Unrestricted Cash and Cash Equivalents in an amount at all
     times not less than (i) One Hundred Twenty Million and 00/100 Dollars
     ($120,000,000.00) from the Closing through March 31, 2003, (ii) Ninety
     Million and 00/100 Dollars ($90,000,000.00) from April 1, 2003 through June
     30, 2003, (iii) Sixty-Five Million and 00/100 Dollars ($65,000,000.00) from
     July 1, 2003 through September 30, 2003; and (iv) Fifty Million and 00/100
     Dollars ($50,000,000.00) on October 1, 2003 and all times thereafter.

             (b) The Borrower and its Subsidiaries, on a consolidated basis,
     shall maintain at all times (i) Unrestricted Cash and Cash Equivalents
     divided by (ii) total liabilities of the Borrower (excluding Subordinated
     Debt), of at least 1.

     5.14    DISSOLUTION OF PHARMACEUTICALS CANADA, INC. AND TERRAGEN DISCOVERY,
INC. Within 60 days from and after the date of this Agreement, Borrower shall
initiate the corporate dissolution of Cubist Pharmaceuticals Canada, Inc. and
TerraGen Discovery, Inc. and shall complete such dissolution as promptly as
possible.

SECTION 6.   NEGATIVE COVENANTS.

     The Borrower hereby covenants and agrees that so long as all or any portion
of the Loan is outstanding, that:

     6.1     LIMITATION ON CHANGES IN FISCAL YEAR. The Borrower shall not, and
shall not permit any of its Subsidiaries to, change the fiscal year of the
Borrower or any of its Subsidiaries.

     6.2     LIMITATION ON INDEBTEDNESS. The Borrower shall not, and shall cause
each of its Subsidiaries not to, create, incur, assume or suffer to exist any
Indebtedness, except:

             (a) Indebtedness to the Lender arising under any of the Financing
     Documents;

             (b) Subordinated Debt;

                                      -20-
<Page>

             (c) in addition to the Indebtedness listed in the other subsections
     of this Section 6.2, Indebtedness of the Borrower and any of its
     Subsidiaries, on a consolidated basis, not to exceed Five Million and
     00/100 Dollars in the aggregate, including, without limitation, Purchase
     Money Indebtedness; Indebtedness with respect to Capitalized Lease
     Obligations; and the Indebtedness with respect to the GE Capital Lease;

             (d) Hancock Indebtedness;

             (e) current liabilities which are incurred in the ordinary course
     of business and which are not incurred through (i) the borrowing of money
     or (ii) the obtaining of credit except for credit on an open account basis
     customarily extended and in fact extended in connection with normal
     purchases of goods and services;

             (f) Indebtedness with respect to taxes, assessments, governmental
     charges or levies which are being contested in good faith by appropriate
     proceedings, provided that adequate reserves with respect thereto are
     maintained on the books of the Borrower or its Subsidiaries, as the case
     may be, in conformity with GAAP; and

             (g) Indebtedness of the Borrower to any Subsidiary and of any
     Subsidiary to the Borrower or any other Subsidiary, so long as such
     Indebtedness (i) is subordinated in right of payment to all Obligations;
     and (ii) has terms and conditions as the Lender may reasonably require.

     6.3     LIMITATION ON CONTINGENT LIABILITIES. The Borrower shall not, and
shall not permit any of its Subsidiaries to, assume, guarantee, endorse or
otherwise become directly or contingently liable (including without limitation,
liable by way of agreement, contingent or otherwise to purchase or provide funds
for payment, to supply funds to or otherwise invest in any debtor or otherwise
to assure any creditor against any loss) in connection with any Indebtedness of
any other Person, except for: (a) liabilities to the Lender arising under any of
the Financing Documents; and (b) guarantees made in the ordinary course of the
business by the Borrower of obligations of any Subsidiary, which obligations are
otherwise permitted under this Agreement.

     6.4     LIMITATION ON LIENS. The Borrower shall not, and shall not permit
any of its Subsidiaries to, create, incur, assume or suffer to exist any Lien
upon any of its property, assets or revenues, whether now owned or hereafter
acquired, except for the following (hereinafter referred to collectively as
"PERMITTED LIENS"):

             (a) Liens created pursuant to the Security Documents;

             (b) Liens for taxes not yet due or which are being contested in
     good faith by appropriate proceedings, provided that adequate reserves with
     respect thereto are maintained on the books of the Borrower or its
     Subsidiaries, as the case may be, in conformity with GAAP;

             (c) statutory landlords' liens and carriers', warehousemen's,
     mechanics', materialmen's, repairmen's or other like Liens arising in the
     ordinary course of business for sums which are not overdue for a period of
     more than sixty (60) days or which are being contested in good faith by
     appropriate proceedings;

                                      -21-
<Page>

             (d) judgment Liens created by or resulting from any litigation or
     legal proceeding if released or bonded within thirty (30) days of the date
     of creation thereof, unless such litigation or legal proceeding could
     reasonably be expected to have a Material Adverse Effect;

             (e) pledges or deposits in connection with workers' compensation,
     unemployment insurance and other social security legislation and deposits
     securing liability to insurance carriers under insurance or self-insurance
     arrangements;

             (f) deposits to secure the performance of bids, trade contracts
     (other than for borrowed money), leases, statutory obligations, surety and
     appeal bonds, performance bonds and other obligations of a like nature
     incurred in the ordinary course of business;

             (g) Liens consisting of easements, zoning restrictions, flowage
     rights, rights-of-way, covenants, conditions, restrictions, reservations,
     licenses, agreements and other similar matters, which, in the aggregate,
     are not substantial in amount and which do not in any case materially
     detract from the use of the property subject thereto or materially
     interfere with the ordinary conduct of the business of the Borrower or such
     Subsidiary;

             (h) Liens to secure Indebtedness for Purchase Money Indebtedness to
     the extent that such Indebtedness is permitted under subsection 6.2(c);
     PROVIDED, HOWEVER, that (i) each such Lien is given only to secure the
     purchase price of the property which is the subject of such Purchase Money
     Indebtedness, does not extend to any other property and is given at the
     time of acquisition of the property; and (ii) the Purchase Money
     Indebtedness secured thereby does not exceed the lesser of the cost of such
     property or its fair market value at the time of acquisition;

             (i) Liens in favor of lessors under Capitalized Leases to the
     extent that the Capitalized Lease Obligations thereunder is Indebtedness
     permitted under subsection 6.2(c); PROVIDED, HOWEVER, that each such Lien
     extends only to the property which is subject of such Capitalized Lease, is
     given only to secure the Capitalized Lease Obligations under such
     Capitalized Lease, and is given at the commencement date of such
     Capitalized Lease; and

             (j) Liens in existence on the date hereof listed on the MASTER
     DISCLOSURE SCHEDULE; PROVIDED, HOWEVER, that no such Lien encumbers any
     additional property after the Closing Date and that the amount of
     Indebtedness secured thereby shall not subsequently be increased.

             (k) Liens in favor of General Electric Capital Corporation pursuant
     to the GE Capital Lease up to an aggregate amount of $2,000,000.

     6.5     LIMITATION ON NEGATIVE PLEDGES. The Borrower shall not, and shall
not permit any of its Subsidiaries to, enter into with any Person any agreement
(other than this Agreement and the other Financing Documents) which prohibits or
limits the ability of the Borrower or any of its Subsidiaries to create, incur,
assume or suffer to exist any Lien upon any of its property, assets or revenues
whether now owned or hereafter acquired, or which prohibits or limits loans or

                                      -22-
<Page>

dividends by any of the Subsidiaries to the Borrower, except for such Liens
which secure Indebtedness permitted under Section 6.2(c).

     6.6     LIMITATION ON FUNDAMENTAL CHANGES. The Borrower shall not, and
shall not permit any of its Subsidiaries to, enter into any merger,
consolidation or amalgamation, or liquidate, wind up or dissolve itself (or
suffer any liquidation or dissolution), or convey, sell, lease, assign, transfer
or otherwise dispose of, all or substantially all of its property, business or
assets, or make any material change in its present method of conducting
business, except:

             (a) any Subsidiary of the Borrower may be merged or consolidated
     with or into the Borrower (provided that the Borrower shall be the
     continuing or surviving corporation) or with or into any one or more wholly
     owned Subsidiaries of the Borrower (provided that the wholly owned
     Subsidiary or Subsidiaries shall be the continuing or surviving
     corporation);

             (b) any wholly owned Subsidiary may sell, lease, transfer or
     otherwise dispose of any or all of its assets (upon voluntary liquidation
     or otherwise) to the Borrower or any other wholly owned Subsidiary of the
     Borrower; and

             (c) pursuant to any sale of assets expressly permitted by
     subsection 6.7.

     6.7     LIMITATION ON SALE OF ASSETS. The Borrower shall not, and shall not
permit any of its Subsidiaries to, convey, sell, lease, assign, transfer or
otherwise dispose of any of its property, business or assets (including, without
limitation, receivables and leasehold interests), whether now owned or hereafter
acquired, or, in the case of any Subsidiary, issue or sell any shares of such
Subsidiary's Capital Stock to any Person (other than the Borrower or any wholly
owned Subsidiary, and subject to the provisions of the Security Documents),
except:

             (a) the conveyance, sale, lease, assignment, transfer or other
     disposition of Obsolete Property or surplus property in the ordinary course
     of business;

             (b) the sale of inventory in the ordinary course of business;

             (c) the sale or discount for fair value, without recourse and
     consistent with sound business practices of accounts receivable arising in
     the ordinary course of business in connection with the compromise or
     collection thereof;

             (d) the license of Intellectual Property in the ordinary course of
     business;

             (e) leases or subleases of property not materially interfering with
     the ordinary course of conduct of the business of the Borrower and its
     Subsidiaries;

             (f) the sale or transfer of property and assets to the extent and
     as permitted by subsection 6.6(b); and

     6.8     LIMITATION ON SALES AND LEASEBACKS. The Borrower shall not, and
shall not permit any of its Subsidiaries to, enter into any arrangement with any
Person providing for the leasing by the Borrower or any Subsidiary of real or
personal property which has been or is to be sold or

                                      -23-
<Page>

transferred by the Borrower or such Subsidiary to such Person or to any other
Person to whom funds have been or are to be advanced by such Person on the
security of such property or rental obligations of the Borrower or such
Subsidiary; PROVIDED that the Borrower may enter into such an arrangement (i) in
the GE Capital Lease, and (ii) with respect to its real property in Lexington,
Massachusetts if (a) the sales price for such real property is not less than
Thirty-Four Million Dollars, (b) the Hancock Indebtedness is repaid in full
contemporaneously with the Borrower entering into such arrangement, and (c) no
Event of Default shall have then occurred and be continuing or would result
therefrom unless the Lender has waived the conditions in clauses (a), (b) and/or
(c) pursuant to Section 9.2.

     6.9     RESTRICTED PAYMENTS. The Borrower shall not, and shall not permit
any of its Subsidiaries to, declare or pay any dividend on, or make any payment
on account of, or set apart assets for a sinking or other analogous fund for,
the purchase, redemption, defeasance, retirement or other acquisition of, any
shares of any class of Capital Stock of the Borrower or any of its Subsidiaries
or any warrants or options to purchase any such shares of Capital Stock, whether
now or hereafter outstanding, or make any other distribution in respect thereof,
either directly or indirectly, whether in cash or property or in obligations of
the Borrower or any of its Subsidiaries (such declarations, payments, setting
apart, purchases, redemptions, defeasances, retirements, acquisitions and
distributions being herein called "RESTRICTED PAYMENTS"), except that,

             (a) so long as no Event of Default, or event which with the passage
     of time, the giving of notice, or both, would constitute an Event of
     Default, has occurred or is continuing, any Subsidiary may declare and pay
     dividends to the Borrower or any other Subsidiary, to the extent and as
     provided under the Security Documents;

             (b) the Borrower shall be permitted to grant, from time to time, to
     its officers, directors and employees options to purchase up to twenty
     percent (20%) of the shares of Capital Stock of the Borrower which, after
     such issuance, would be outstanding on a fully-diluted and fully-converted
     basis, so long as no Event of Default, or event which with the passage of
     time, the giving of notice, or both, would constitute an Event of Default,
     has occurred or is continuing;

             (c) the Borrower may repurchase, redeem or otherwise acquire or
     retire for value any shares of Capital Stock held by officers, directors
     and employees of the Borrower pursuant to any employee equity subscription
     agreement, stock option agreement or stock ownership arrangement; provided
     that (i) the aggregate price paid for all such repurchased, redeemed,
     acquired or retired shares of Capital Stock shall not exceed One Hundred
     Thousand and 00/100 Dollars ($100,000.00) in any twelve-month period plus
     the aggregate cash proceeds received by the Borrower during such
     twelve-month period from any reissuance of shares of Capital Stock to
     employees of the Borrower; and (ii) no Event of Default shall have then
     occurred and be continuing or would result therefrom, provided that this
     clause (ii) shall not prohibit any transaction under this subsection clause
     (c) within sixty (60) days of the date of declaration or the making of any
     binding commitment in respect of any such transaction if at said date of
     declaration or commitment no Event of Default shall have then occurred and
     be continuing or would result therefrom.

                                      -24-
<Page>

             (d) the Borrower may repay the Subordinated Debt in its sole
     discretion, provided that (i) immediately after such repayment the Borrower
     and its Subsidiaries, on a consolidated basis, shall maintain Unrestricted
     Cash and Cash Equivalents in an amount not less than One Hundred Twenty
     Million and 00/100 Dollars ($120,000,000), and (ii) no Event of Default
     shall have then occurred and be continuing or would result therefrom,
     except that this clause shall not prohibit any transaction under this
     subsection clause (d) within sixty (60) days of the date of declaration or
     the making of any binding commitment in respect of any such transaction if
     at said date of declaration or commitment the Borrower is in compliance
     with (i) and (ii) hereof.

     6.10    LIMITATION ON INVESTMENTS, LOANS AND ADVANCES. The Borrower shall
not, and shall not permit any of its Subsidiaries to, make any advance, loan,
extension of credit or capital contribution to, or purchase any stock, bonds,
notes, debentures or other securities of, or any assets constituting a business
unit of, or make any other investment in, any Person (an "INVESTMENT"), except:

             (a) investments in Cash Equivalents;

             (b) securities held by the Borrower or any of its Subsidiaries
     prior to the Closing Date and listed on the MASTER DISCLOSURE SCHEDULE;

             (c) Investments by the Borrower in any Subsidiary and Investments
     by any such Subsidiary in the Borrower or in any other Subsidiary;

             (d) investments made pursuant to any strategic alliance, joint
     venture, corporate partnering arrangement, research collaboration or
     development, marketing, manufacturing, distribution or licensing
     arrangement that is entered into by the Borrower or any Subsidiaries;
     provided that such transaction or arrangement is entered into in the
     ordinary course of business and no Event of Default shall have then
     occurred and be continuing or would result therefrom; and

             (e) extensions of trade credit and endorsements of negotiable
     instruments and other negotiable documents in the ordinary course of
     business.

     6.11    LIMITATION ON TRANSACTIONS WITH AFFILIATES. The Borrower shall not,
and shall not permit any of its Subsidiaries to, enter into any transaction
(including, without limitation, any purchase, sale, lease or exchange of
property or the rendering of any service) with any Affiliate unless such
transaction is (a) otherwise permitted under this Agreement; (b) in the ordinary
course of the Borrower's or such Subsidiary's business; and (c) upon fair and
reasonable terms no less favorable to the Borrower or such Subsidiary, as the
case may be, than it would obtain in a comparable arm's length transaction with
a Person which is not an Affiliate; PROVIDED, HOWEVER, that the foregoing
restriction shall not prohibit (i) any employment agreement entered into by the
Borrower or any of its Subsidiaries in the ordinary course of business; (ii) any
issuance of securities in connection with employment arrangements, stock options
and stock ownership plans of the Borrower entered into in the ordinary course of
business; (iii) transactions between the Borrower and its Subsidiaries; and (iv)
the transactions contemplated by the agreements listed on the MASTER DISCLOSURE
SCHEDULE.

                                      -25-
<Page>

     6.12    LIMITATION ON LINES OF BUSINESS. The Borrower shall not, and shall
not permit any of its Subsidiaries to, enter into any business, either directly
or through any Subsidiary, except for those businesses in which the Borrower and
its Subsidiaries are engaged on the date of this Agreement or which are
reasonably related thereto.

     6.13    NEGATIVE PLEDGE ON INTELLECTUAL PROPERTY. The Borrower shall not,
and shall not permit any of its Subsidiaries to, create, incur, assume or suffer
to exist any Lien upon the Intellectual Property of the Borrower or any of its
Subsidiaries, whether now owned or hereafter acquired, except as set forth in
the Security Agreements.

     6.14    NEGATIVE PLEDGE ON FOREIGN SUBSIDIARY ASSETS. The Borrower shall
not, and shall not permit any of its Subsidiaries to, create, incur, assume or
suffer to exist any Lien upon any of the assets (including, without limitation,
receivables and leasehold interests) of any of its Foreign Subsidiaries, except
as set forth in the Security Agreements.

     6.15    LIMITATION ON CURRENT MATURITIES OF INDEBTEDNESS. The Borrower
shall not, and shall not permit any of its Subsidiaries, on a consolidated
basis, to make scheduled annual principal payments on any Indebtedness in excess
of Six Million and 00/100 Dollars ($6,000,000.00) in the aggregate, or allow the
creation or existence of Indebtedness that would result in current maturities of
Indebtedness being in excess of Six Million and 00/100 Dollars ($6,000,000.00),
PROVIDED that any repayment of the Subordinated Debt pursuant to Section 6.9(d)
shall be excluded from principal payments on Indebtedness for purposes of this
Section 6.15.

     6.16    LIMITATION ON TRANSFERS TO CUBIST PHARMACEUTICALS, INC. AND
TERRAGEN DISCOVERY, INC. The Borrower shall not, and shall not permit any of its
Subsidiaries to, sell, transfer or convey any Collateral to Cubist
Pharmaceuticals Canada, Inc. or TerraGen Discovery, Inc.

SECTION 7.   EVENTS OF DEFAULT.

     7.1     EVENTS OF DEFAULT; ACCELERATION. If any of the following events
             shall occur:

             (a) the Borrower shall fail to pay any principal of the Loan when
     due in accordance with the terms of this Agreement and the other Financing
     Documents; or the Borrower shall fail to pay any interest on any Loan, or
     any other amount payable hereunder; or

             (b) any representation or warranty made or deemed made by the
     Borrower or any other Loan Party herein or in any other Financing Document
     or which is contained in any certificate, document or financial or other
     statement furnished by it at any time under or in connection with this
     Agreement or any such other Financing Document shall prove to have been
     incorrect in any material respect on or as of the date made or deemed made;
     or

             (c) the failure by the Borrower or any other Loan Party to
     punctually perform, observe, comply with or satisfy any covenant, agreement
     or condition contained in (i) Section 5 or Section 6 of this Agreement or
     (ii) Section 4.1, 5.4, 5.5, 5.6(a)-(c) of each of the Security Agreements,
     which failure is not cured within ten (10) days; or

                                      -26-
<Page>

             (d) the Borrower or any Subsidiary shall default in the observance
     or performance of any other agreement contained in this Agreement or any
     other Financing Document (other than as provided in paragraphs (a) through
     (c) of this Section), and such default shall continue unremedied for a
     period of thirty (30) days after the earlier of (i) the date on which a
     Responsible Officer of the Borrower first learns of such default or (ii)
     the date on which written notice thereof shall have been given to the
     Borrower by the Lender; or

             (e) the Borrower or any Subsidiary shall fail to pay when due
     (after any applicable period of grace) any Indebtedness of the Borrower or
     such Subsidiary (other than (i) Indebtedness comprising the Obligations and
     (ii) Subordinated Debt), which together with all such other due but unpaid
     Indebtedness, exceeds the sum of One Hundred Thousand and 00/100 Dollars
     ($100,000.00), or shall fail (after any applicable period of grace) to
     observe or perform or obtain a waiver with respect to any term, covenant or
     agreement evidencing or securing such Indebtedness, which, if uncured or
     unwaived, permits the acceleration of such Indebtedness, or any default or
     event of default shall have been declared under any agreement relating to
     such Indebtedness; or

             (f) the Borrower or any other Loan Party shall (i) apply for or
     consent to the appointment of, or the taking of possession by, a receiver,
     custodian, trustee, liquidator or similar official of itself or of all or a
     substantial part of its property, (ii) be generally not paying its debts as
     such debts become due (other than nonpayment of specific debts because of
     bona fide disputes), (iii) make a general assignment for the benefit of its
     creditors, (iv) commence a voluntary case under the United States
     Bankruptcy Code, as amended from time to time, (v) take any action or
     commence any case or proceeding under any law relating to bankruptcy,
     insolvency, reorganization, winding-up or composition or adjustment of
     debts, or any other law providing for the relief of debtors, (vi) fail to
     contest in a timely or appropriate manner, or acquiesce in writing to, any
     petition filed against it in an involuntary case under the United States
     Bankruptcy Code, as amended from time to time or other law, (vii) take any
     action under the laws of its jurisdiction of incorporation or organization
     similar to any of the foregoing, or (viii) take any corporate action for
     the purpose of effecting any of the foregoing; or

             (g) a proceeding or case shall be commenced against the Borrower or
     any other Loan Party, without the application or consent of the Borrower or
     such Loan Party, in any court of competent jurisdiction, seeking (i) the
     liquidation, reorganization, dissolution, winding up, or composition or
     readjustment of its debts, (ii) the appointment of a trustee, receiver,
     custodian, liquidator or the like of it or of all or any substantial part
     of its assets, or (iii) similar relief in respect of it, under any law
     relating to bankruptcy, insolvency, reorganization, winding-up or
     composition or adjustment of debts or any other law providing for the
     relief of debtors, and such proceeding or case shall continue undismissed,
     or unstayed and in effect, for a period of sixty (60) days; or an order for
     relief shall be entered in an involuntary case under the United States
     Bankruptcy Code, as amended from time to time, against the Borrower or any
     other Loan Party; or action under the laws of the jurisdiction of
     incorporation or organization of the Borrower or any other Loan Party
     similar to any of the foregoing shall be taken with respect to the Borrower
     or any other Loan Party and shall continue unstayed and in effect for a
     period of sixty (60) days; or

                                      -27-
<Page>

             (h) (i) the Borrower or any Commonly Controlled Entity shall fail
     to pay when due any amount that it shall have become liable to pay to the
     PBGC or to a Plan under Title IV of ERISA, unless (A) such liability is
     being contested in good faith by appropriate proceedings, the Borrower or
     such Commonly Controlled Entity, as the case may be, has established and is
     maintaining adequate reserves in accordance with GAAP and no lien shall
     have been filed to secure such liability or (B) which would not have a
     Material Adverse Effect; (ii) the PBGC shall institute proceedings under
     Title IV of ERISA to terminate or to cause a trustee to be appointed to
     administer any such Plan or Plans; or (iii) a condition shall exist by
     reason of which the PBGC would be entitled to obtain a decree adjudicating
     that any such Plan or Plans must be terminated; or

             (i) one or more judgments or decrees shall be entered against the
     Borrower or any of its Subsidiaries involving individually a liability of
     Two Hundred and Fifty Thousand and 00/100 Dollars ($250,000.00) (not paid
     or fully covered by insurance) or in the aggregate a liability (not paid or
     fully covered by insurance) of Five Hundred Thousand and 00/100 Dollars
     ($500,000.00) or more, and all such judgments or decrees shall not have
     been vacated, discharged, stayed or bonded pending appeal within thirty
     (30) days from the entry thereof; or

             (j) service of any process upon the Lender, seeking to attach by
     Lien, levy, mesne, trustee or other process, any funds in excess of Five
     Hundred Thousand and 00/100 Dollars ($500,000.00)of the Borrower or any of
     its Subsidiaries on deposit with, or in possession or control of the
     Lender; or

             (k) if any of the Financing Documents (including the Subsidiary
     Guaranty) (or any provision contained therein) shall be cancelled,
     terminated, revoked, curtailed or rescinded otherwise than in accordance
     with the terms thereof or with the express prior written agreement, consent
     or approval of the Lender, or any action at law, suit or in equity or other
     legal proceeding to cancel, revoke, curtail or rescind any of the Financing
     Documents shall be commenced by or on behalf of the Borrower or any of its
     officers, director or stockholders, or any Governmental Authority of
     competent jurisdiction shall make a determination that, or issue a
     judgment, order, decree or ruling to the effect that, any of the Financing
     Documents (including the Subsidiary Guaranty) (or any provision contained
     therein) is illegal, invalid or unenforceable in accordance with the terms
     thereof; or

             (l) any of the Security Documents shall, at any time after their
     execution and delivery for any reason, cease to create a valid and
     perfected first priority security interest in and to all of the Collateral
     pledged or granted thereunder; or

             (m) a material portion of the property of the Borrower and its
     Subsidiaries (whether or not Collateral) is damaged by fire or other
     casualty, or otherwise lost or stolen, the restoration or replacement cost
     of which property exceeds, in the aggregate, the amount of insurance
     proceeds readily available for such restoration or replacement, and such
     loss would have a Material Adverse Effect; or

                                      -28-
<Page>

             (n) any default shall exist and remains unwaived or uncured with
     respect to any of the Subordinated Debt if, as a result of such default,
     any holder of the Subordinated Debt, is entitled to cause any such
     Subordinated Debt to become due prior to its stated date of maturity; or

             (o) any of the subordination provisions contained in any of the
     Subordination Agreements ceases to be enforceable in accordance with its
     terms;

then, and in any such event, so long as the same may be continuing, the Lender
may, by notice in writing to the Borrower, declare all amounts owing with
respect to this Agreement, the Notes and the other Financing Documents to be,
and they shall thereupon forthwith become, immediately due and payable without
presentment, demand, protest or other notice of any kind, all of which are
hereby expressly waived by the Borrower; provided that in the event of any Event
of Default specified in subsection 7.1(f) or subsection 7.1(g), all such amounts
shall become immediately due and payable automatically and without any
requirement of notice from the Lender.

SECTION 8.   RIGHTS AND REMEDIES.

     8.1     RIGHTS AND REMEDIES. In case any one or more of the Events of
Default shall have occurred and be continuing, and whether or not the Lender
shall have accelerated the maturity of the Loan pursuant to subsection 7.1, the
Lender, if owed any amount with respect to the Loan may proceed to protect and
enforce its rights by suit in equity, action at law or other appropriate
proceeding, whether for the specific performance of any covenant or agreement
contained in this Agreement and the other Loan Documents or any instrument
pursuant to which the Obligations to the Lender are evidenced, including as
permitted by applicable law the obtaining of the ex parte appointment of a
receiver, and, if such amount shall have become due, by declaration or
otherwise, proceed to enforce the payment thereof or any other legal or
equitable right of the Lender.

     8.2     SETOFF. Regardless of the adequacy of any of the Collateral,
upon the occurrence and during the continuance of any Event of Default, any
deposits or other sums credited by or due from the Lender to the Borrower and
any securities or other property of the Borrower in the possession of the Lender
may be applied to or set off by the Lender against the payment of Obligations
and any and all other liabilities, direct, or indirect, absolute or contingent,
due or to become due, now existing or hereafter arising, of the Borrower to the
Lender.

     8.3     NO WAIVER; CUMULATIVE REMEDIES. No failure to exercise and no
delay in exercising, on the part of the Lender, any right, remedy, power or
privilege hereunder or under the other Financing Documents shall operate as a
waiver thereof; nor shall any single or partial exercise of any right, remedy,
power or privilege hereunder preclude any other or further exercise thereof or
the exercise of any other right, remedy, power or privilege. The rights,
remedies, powers and privileges herein provided are cumulative and not exclusive
of any rights, remedies, powers and privileges provided by law.

     8.4     DISTRIBUTION OF COLLATERAL PROCEEDS. In the event that, following
the occurrence or during the continuance of any Event of Default, the Lender
receives any monies in connection

                                      -29-
<Page>

with the enforcement of any of the Security Documents, or otherwise with respect
to the realization upon any of the Collateral, such monies shall be distributed
for application as follows: (a) FIRST, to the Obligations in such order or
preference as the Lender may determine; (b) SECOND, upon payment and
satisfaction in full or other provisions for payment in full satisfactory to the
Lender of all of the Obligations, to the payment of any obligations required to
be paid pursuant to Section 9-504(l)(c) of the UCC; and (c) THIRD, the excess,
if any, shall be returned to the Borrower or to such other Persons as are
entitled thereto.

SECTION 9.   MISCELLANEOUS.

     9.1     SURVIVAL OF COVENANTS. Except for those which by their terms
survive termination of the Financing Documents, all agreements, representations,
covenants and warranties made by the Borrower and any Loan Party in the
Financing Documents shall remain in full force and effect until all Obligations
to the Lender have been paid in full and satisfied.

     9.2     PRIOR DISCUSSIONS; AMENDMENTS IN WRITING; COUNTERPARTS. The
Financing Documents incorporate all discussions and negotiations among the
Lender, the Borrower and the other Loan Parties and either express or implied,
concerning the Obligations, notwithstanding any custom, usage or oral agreement
or understanding to the contrary. This Agreement may be amended or modified only
in writing signed by the parties hereto, and in the case of the Lender signed by
a duly authorized officer thereof. This Agreement may be executed in two or more
counterparts, each of which shall constitute an original, but such counterparts
together shall constitute one and the same instrument. Any proof of this
Agreement shall require production of only one such counterpart.

     9.3     SEVERABILITY. Any provision of this Agreement which is
prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction,
be ineffective to the extent of such prohibition or unenforceability without
invalidating the remaining provisions hereof, and any such prohibition or
unenforceability in any jurisdiction shall not invalidate or render
unenforceable such provision in any other jurisdiction.

     9.4     NOTICES. All notices, requests and demands to or upon the
respective parties hereto to be effective shall be in writing (including by
facsimile transmission), and, unless otherwise expressly provided herein, shall
be deemed to have been duly given when delivered by hand, or when sent by
facsimile transmission or by telex, answer back received, or on the first
Business Day after delivery to any overnight delivery service, freight prepaid,
or three (3) Business Days after being sent by certified or registered mail,
return receipt requested, postage prepaid, and addressed as follows in the case
of the Borrower, any Subsidiary and the Lender, or to such other address as may
be hereafter notified by the respective parties hereto:

             (a) If to the Borrower
                  or any Subsidiary, then:  Cubist Pharmaceuticals, Inc.
                                            65 Hayden Avenue
                                            Lexington, MA 02421
                                            Attention: Christopher D.T.
                                            Guiffre, Esq., Vice President,
                                            General Counsel & Secretary
                                            Telecopier No: (781) 861-0566

                                      -30-
<Page>

                           with copies to:  Bingham McCutchen
                                            150 Federal Street
                                            Boston, Massachusetts 02110
                                            Attention: Julio E. Vega, Esq.
                                            Telecopier No.: (617) 951-8736

             (b) If to the Lender, then:    Citizens Bank of Massachusetts
                                            53 State Street
                                            Boston, Massachusetts 02109
                                            Attention: Scott Haskell,
                                            Vice President
                                            Telecopier No: (617) 742-9548

                           with copies to:  Eckert Seamans Cherin & Mellott, LLC
                                            One International Place
                                            Boston, MA  02110
                                            Attention: Anil Khosla, Esq.
                                            Telecopier No: 617-342-6899

provided that any notice, request or demand to or upon the Lender pursuant to
subsection 2.2, 2.6, 2.7 or 2.8 shall not be effective until received.

     9.5     EXPENSES. The Borrower agrees to pay (a) the reasonable costs
of producing and reproducing this Agreement, the other Financing Documents and
the other agreements and instruments mentioned herein, (b) any taxes (including
any interest and penalties in respect thereto) payable by the Lender (other than
taxes based upon the Lender's net income) on or with respect to the transactions
contemplated by this Agreement (the Borrower hereby agreeing to indemnify the
Lender with respect thereto), (c) the reasonable fees, expenses and
disbursements of counsel to the Lender incurred in connection with the
preparation, negotiation, administration or interpretation of the Financing
Documents and other instruments mentioned herein, and amendments, modifications,
approvals, consents or waivers hereto or hereunder, (d) the reasonable fees,
expenses and disbursements of the Lender incurred by the Lender in connection
with the preparation, negotiation, administration or interpretation of the
Financing Documents and other instruments mentioned herein, including all title
insurance premiums and surveyor, engineering and appraisal charges, (e) all
reasonable out-of-pocket expenses (including without limitation reasonable
attorneys' fees and costs of outside legal counsel, and reasonable consulting,
accounting, appraisal, investment banking and similar professional fees and
charges) incurred by the Lender in connection with (i) the enforcement of or
preservation of rights under any of the Financing Documents against the Borrower
or the administration thereof after the occurrence of a Default or Event of
Default (including engineering appraiser and investment banking charges) and
(ii) any litigation, proceeding or dispute whether arising hereunder or
otherwise, in any way related to the Lender's relationship with the Borrower and
(f) all reasonable fees, expenses and disbursements of the Lender incurred in
connection with UCC searches, UCC filings or mortgage recordings. The covenants
contained in this subsection shall survive payment or satisfaction in full of
all other Obligations.

                                      -31-
<Page>

     9.6     INDEMNIFICATION. The Borrower agrees to indemnify and hold
harmless the Lender from and against any and all claims, actions and suits
whether groundless or otherwise, and from and against any and all liabilities,
losses, damages and expenses of every nature and character arising out of this
Agreement or any of the other Financing Documents or the transactions
contemplated hereby including, without limitation, (a) any actual or proposed
use by the Borrower of the proceeds of the Loan, (b) any actual or alleged
infringement of any patent, copyright, trademark, service mark or similar right
of the Borrower or any other Loan Party comprised in the Collateral, (c) all
liabilities, obligations, claims, damages, costs, losses and expenses (including
court costs and attorney's reasonable fees and expenses) that the Lender may
sustain or incur by reason of, relating to or arising out of the preparation of
this Agreement, the defending or protecting of any Collateral or the priority of
the Lender's interest therein, or in collecting or enforcing the Obligations, or
in enforcing any of the Lender's rights or remedies, or in the prosecution or
defense of any action or proceeding concerning any matter growing out of or
connected with this Agreement, any of the other Financing Documents, the
Obligations, the Collateral, or on account of the Lender's relationship with the
Borrower or any other Loan Party (except for such claims which have been
determined by a court of competent jurisdiction to have arisen out of the
Lender's actual bad faith, willful misconduct or gross negligence) or (d) with
respect to the Borrower or any other Loan Party and their respective properties
and assets, the violation of any Environmental Law, the presence, disposal,
escape, seepage, leakage, spillage, discharge, emission, release or threatened
release of any Hazardous Substances or any action, suit, proceeding or
investigation brought or threatened with respect to any hazardous Substances
(including, but not limited to, claims with respect to wrongful death, personal
injury or damage to property), in each case including, without limitation, the
reasonable fees and disbursements of counsel and allocated costs of internal
counsel incurred in connection with any such investigation, litigation or other
proceeding; PROVIDED HOWEVER that such indemnification shall not apply to claims
which have been determined by a court of competent jurisdiction to have arisen
out of the Lender's actual bad faith, willful misconduct or gross negligence. In
litigation, or the preparation therefor, the Lender shall be entitled to select
its own counsel and, in addition to the foregoing indemnity, the Borrower agrees
to pay promptly the reasonable fees and expenses of such counsel. If, and to the
extent that the obligations of the Borrower under this subsection are
unenforceable for any reason, the Borrower hereby agrees to make the maximum
contribution to the payment in satisfaction of such obligations which is
permissible under applicable law. The covenants contained in this subsection
shall survive payment or satisfaction in full of all other Obligations.

     9.7     ACKNOWLEDGEMENTS. The Borrower hereby acknowledges that (a) the
Borrower has been advised by counsel in the negotiation, execution and delivery
of this Agreement and the other Financing Documents; (b) the Lender has no
fiduciary relationship with or duty to the Borrower arising out of or in
connection with this Agreement or any of the other Financing Documents, and the
relationship of the Lender, on one hand, and the Borrower, on the other hand, in
connection herewith or therewith is solely that of debtor and creditor; and (c)
no joint venture is created hereby or by the other Financing Documents or
otherwise exists by virtue of the transactions contemplated hereby between the
Borrower and the Lender.

     9.8     SUCCESSORS AND ASSIGNS. This Agreement shall be binding upon
and inure to the benefit of the Borrower, the Lender and their respective
successors and assigns, except that the Borrower may not assign or transfer any
of its rights or obligations under this Agreement.

                                      -32-
<Page>

     9.9     LOSS, THEFT, DESTRUCTION OR MUTILATION OF ANY NOTE. Upon
receipt of an affidavit of an officer of the Lender as to the loss, theft,
destruction or mutilation of any Note or any other Financing Document which is
not of public record, and, in the case of any such loss, theft, destruction or
mutilation, upon cancellation of such Note or other Financing Document, the
Borrower will issue, in lieu thereof, a replacement note or other Financing
Document in the same principal amount thereof and otherwise of like tenor.

     9.10    WAIVER OF JURY TRIAL. THE BORROWER AND THE LENDER HEREBY WAIVE
THEIR RESPECTIVE RIGHTS TO A TRIAL BY JURY OF ANY CLAIM OR CAUSE OF ACTION BASED
UPON OR ARISING OUT OF THIS AGREEMENT, ANY OF THE OTHER FINANCING DOCUMENTS OR
ANY OF THE TRANSACTIONS CONTEMPLATED HEREIN OR THEREIN, INCLUDING CONTRACT
CLAIMS, TORT CLAIMS, BREACH OF DUTY CLAIMS AND ALL OTHER COMMON LAW OR STATUTORY
CLAIMS.

     9.11    GOVERNING LAW; JURISDICTION. This Agreement and the other
Financing Documents are executed and delivered under seal and shall be construed
in accordance with and governed by the laws of The Commonwealth of
Massachusetts, without giving effect to the conflict of law provisions thereof.
Each of the Lender and the Borrower submits itself to the non-exclusive
jurisdiction of the courts of The Commonwealth of Massachusetts for all purposes
with respect to the Financing Documents and the Borrower's relationship with the
Lender.

            [THE REMAINDER OF THIS PAGE IS LEFT INTENTIONALLY BLANK]

                                      -33-
<Page>

     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed and delivered under seal by their proper and duly authorized
officers as of the day and year first above written.

WITNESS:                           CUBIST PHARMACEUTICALS, INC.

                                   By: /s/ David W. J. McGirr
-------------------------------       --------------------------------
Name:                                 David W. J. McGirr, Senior Vice President
                                      and Chief Financial Officer

WITNESS:                           CITIZENS BANK OF MASSACHUSETTS

                                   By: /s/ Scott Haskell
------------------------------        --------------------------------
Name:                                 Scott Haskell, Vice President

                                      -34-
<Page>

                          COMMONWEALTH OF MASSACHUSETTS

Middlesex County, ss.                                   April __, 2003

     Then personally appeared the above-named Stuart Sohn, Corporate Controller
of Cubist Pharmaceuticals, Inc., and acknowledged the foregoing instrument to be
his free act and deed and the free act and deed of Cubist Pharmaceuticals, Inc.,
before me.

                                          -------------------------------------
                                          Notary Public
                                          My commission expires:
                                          [AFFIX NOTARIAL SEAL]

                          COMMONWEALTH OF MASSACHUSETTS

Suffolk County, ss.                                     April __, 2003

     Then personally appeared the above-named Scott Haskell as Vice President of
Citizens Bank of Massachusetts, and acknowledged the foregoing instrument to be
his free act and deed and the free act and deed of Citizens Bank of
Massachusetts, before me.

                                          -------------------------------------
                                          Notary Public
                                          My commission expires:
                                          [AFFIX NOTARIAL SEAL]

                                      -35-
<Page>

--------------------------------------------------------------------------------

                                CREDIT AGREEMENT
                                (the "AGREEMENT")

                                 by and between

                         CITIZENS BANK OF MASSACHUSETTS
                                 (the "LENDER")

                                       and

                          CUBIST PHARMACEUTICALS, INC.
                                (the "BORROWER")

                           MASTER DISCLOSURE SCHEDULE

--------------------------------------------------------------------------------

     The Borrower represents and warrants to the Lender that the statements
contained in Section 4 and Section 6 of the Agreement are true, correct and
complete as of the date of the Agreement, except as set forth in this Master
Disclosure Schedule (as the same may be supplemented, from time to time, the
"MASTER DISCLOSURE SCHEDULE"). The Master Disclosure Schedule is arranged in
sections corresponding to the lettered and numbered sections contained in
Section 4 and Section 6 of the Agreement.

<Page>

--------------------------------------------------------------------------------

                                CREDIT AGREEMENT
                                (the "AGREEMENT")

                                 by and between

                         CITIZENS BANK OF MASSACHUSETTS
                                 (the "LENDER")

                                       and

                          CUBIST PHARMACEUTICALS, INC.
                                (the "BORROWER")

--------------------------------------------------------------------------------

                                   APPENDIX A

     1.      DEFINITIONS. As used in the Agreement, the following terms shall
have the following meanings:

             "AFFILIATE": as to any Person, any other Person (other than a
     Subsidiary) which, directly or indirectly, is in control of, is controlled
     by, or is under common control with, such Person. For purposes of this
     definition, "control" of a Person means the power, directly or indirectly,
     either to (a) vote 10% or more of the securities having ordinary voting
     power for the election of directors of such Person or (b) direct or cause
     the direction of the management and policies of such Person, whether by
     contract or otherwise.

             "AGREEMENT": this Credit Agreement, as amended, supplemented or
     otherwise modified from time to time.

             "ASSET SALE": with respect each of the Loan Parties, any voluntary
     or involuntary sale or other disposition subsequent to the Closing Date of
     any Collateral.

             "BOARD OF GOVERNORS": the Board of Governors of the Federal Reserve
     System and any Governmental Authority which succeeds to the powers and
     functions thereof.

             "BORROWER": as defined in the preamble to this Agreement.

             "BORROWER SECURITY AGREEMENT": the Borrower Security Agreement to
     be executed and delivered by the Borrower, substantially in the form of
     EXHIBIT B, as the same may be amended, supplemented or otherwise modified
     from time to time.

             "BORROWING DATE": any Business Day specified in a notice pursuant
     to subsection 2.2 as a date on which the Borrower requests the Lender to
     make Loans hereunder.

<Page>

             "BUSINESS DAY": (a) for all purposes other than as described by
     clause (b) below, any day excluding Saturday, Sunday and any day which is a
     legal holiday under the laws of The Commonwealth of Massachusetts, or is a
     day on which banking institutions located in The Commonwealth of
     Massachusetts are required or authorized by any Requirement of Law to be
     closed, and (b) with respect to all notices, determinations, fundings and
     payments in connection with LIBOR Loans, any day which is a Business Day
     described in clause (a) and which is also a day for trading in dollar
     deposits by and between banks in the London interbank market.

             "CAPITAL STOCK": any and all shares, interests, participations or
     other equivalents (however designated) of capital stock of a corporation,
     any and all equivalent ownership interests in a Person (other than a
     corporation) and any and all warrants or options to purchase any of the
     foregoing.

             "CAPITALIZED LEASE": any lease of property, real or personal, the
     obligations of the lessee in respect of which are required in accordance
     with GAAP to be capitalized on a balance sheet of the lessee.

             "CAPITALIZED LEASE OBLIGATIONS": as to any Person, the obligations
     of such Person to pay rent or other amounts under any Capitalized Leases;
     the amount of such obligations at any time shall be the capitalized amount
     thereof at such time determined in accordance with GAAP.

             "CASH EQUIVALENTS": (a) securities issued or directly and fully
     guaranteed or insured by the United States Government, or any agency or
     instrumentality thereof, having maturities of not more than one year from
     the date of acquisition, (b) marketable general obligations issued by any
     state of the United States of America or any political subdivision of any
     such state or any public instrumentality thereof maturing within one year
     from the date of acquisition thereof and, at the time of acquisition
     thereof, having a credit rating of "A" or better from either Standard &
     Poor's Ratings Group or Moody's Investors Service, Inc.; (c) certificates
     of deposit, time deposits, eurodollar time deposits, overnight bank
     deposits or bankers' acceptances having maturities of not more than one
     year from the date of acquisition thereof of the Lender, or of any domestic
     commercial bank the long-term debt of which is rated at the time of
     acquisition thereof at least A or the equivalent thereof by Standard &
     Poor's Ratings Group, or A or the equivalent thereof by Moody's Investors
     Service, Inc., and having capital and surplus in excess of Five Hundred
     Million and 00/100 Dollars ($500,000,000.00), (d) repurchase obligations
     with a term of not more than seven days for underlying securities of the
     types described in clauses (a), (b) and (c) entered into with any bank
     meeting the qualifications specified in clause (c) above, (e) commercial
     paper rated at the time of acquisition thereof at least A-2 or the
     equivalent thereof by Standard & Poor's Ratings Group or P-2 or the
     equivalent thereof by Moody's Investors Service, Inc., or carrying an
     equivalent rating by a nationally recognized rating agency, if both of the
     two named rating agencies cease publishing ratings of investments, and in
     either case maturing within three hundred sixty five (365) days after the
     date of acquisition thereof and (f) other investment instruments approved
     in writing by the Lender and offered by the Lender or by any financial

                                       -2-
<Page>

     institution which has a combined capital and surplus of not less than One
     Hundred Million and 00/100 Dollars ($100,000,000.00).

             "CHARTER DOCUMENTS": as to any Person, the Certificate (or
     Articles) of Incorporation (or Organization) and By-laws or other
     organizational or governing documents of such Person.

             "CLOSING DATE": the date on which the conditions precedent set
     forth in Section 3 shall be satisfied or waived.

             "COLLATERAL": all assets of the Loan Parties, now owned or
     hereinafter acquired, upon which a Lien is purported to be created by any
     Security Document.

             "COMMONLY CONTROLLED ENTITY": an entity, whether or not
     incorporated, which is under common control with the Borrower within the
     meaning of Section 4001 of ERISA or is part of a group which includes the
     Borrower and which is treated as a single employer under Section 414(b) or
     (c) of the Tax Code or, solely for purposes of determining liability under
     Section 412 of the Tax Code, which is treated as a single employer under
     Section 414(b), (c), (m) or (o) of the Tax Code.

             "COMPLIANCE CERTIFICATE": as defined in subsection 5.1(c).

             "CONTRACTUAL OBLIGATION": as to any Person, any provision of any
     security issued by such Person or of any agreement, instrument or other
     undertaking to which such Person is a party or by which it or any of its
     property is bound.

             "DEFAULT": any of the events specified in subsection 7.1, whether
     or not any requirement contained therein for the giving of notice, the
     lapse of time or both, has been satisfied.

             "DESIGNATED SENIOR INDEBTEDNESS": as defined in the Indenture.

             "DOLLARS" and "$": dollars in lawful currency of the United States
     of America.

             "DOMESTIC SUBSIDIARY": any Subsidiary that is organized under the
     laws of any jurisdiction within the United States.

             "ENVIRONMENTAL LAWS": any and all foreign, Federal, state, local or
     municipal laws, rules, orders, regulations, statutes, ordinances, codes,
     decrees, requirements of any Governmental Authority or other Requirements
     of Law (including common law) regulating, relating to or imposing liability
     or standards of conduct concerning protection of human health or the
     environment, as now or may at any time hereafter be in effect.

             "ENVIRONMENTAL PERMITS": all permits, licenses, registrations,
     notifications, exemptions, and other authorizations required under
     Environmental Laws.

             "ERISA": the Employee Retirement Income Security Act of 1974, as
     amended from time to time.

                                       -3-
<Page>

             "EVENT OF DEFAULT": any of the events specified in subsection 7.1,
     PROVIDED, HOWEVER, that any requirement contained therein for the giving of
     notice, the lapse of time or both, has been satisfied.

             "EXTENSION OF CREDIT": the making of any Loan by the Lender.

             "FINANCING DOCUMENTS": this Agreement, the Note, the Security
     Documents, the Subsidiary Guaranty, the Subordination Agreements, and any
     and all other agreements, guaranties, instruments, documents, certificates,
     financing statements, powers of attorney, consents and filings, whether
     heretofore, now, or hereafter executed by or on behalf of the Borrower or
     any of its Subsidiaries or any other Person and delivered to the Lender in
     connection with the Loan, all as may be amended, modified, supplemented,
     restated or extended from time to time.

             "FOREIGN SUBSIDIARY": Any Subsidiary that is organized under the
     laws of any jurisdiction outside the United States.

             "GAAP": generally accepted accounting principles in the United
     States of America in effect from time to time.

             "GE CAPITAL LEASE": the lease arrangement entered into by and
     between Borrower and General Electric Capital Corporation pursuant to that
     certain Master Lease Agreement dated as of March 5, 2003.

             "GOVERNMENTAL AUTHORITY": any nation or government, any state or
     other political subdivision thereof and any entity exercising executive,
     legislative, judicial, regulatory or administrative functions of or
     pertaining to government.

             "HANCOCK INDEBTEDNESS": the Indebtedness of the Borrower pursuant
     to that certain Note Purchase Agreement, dated September 8, 2000, pursuant
     to which the Borrower issued promissory notes in the aggregate original
     principal amount of $39,000,000.

             "HAZARDOUS MATERIALS": any petroleum (including crude oil or any
     fraction thereof) or petroleum products, polychlorinated biphenyls,
     urea-formaldehyde insulation, asbestos and asbestos-containing materials,
     pollutants, contaminants, and all other materials and substances including
     but not limited to radioactive materials regulated pursuant to any
     Environmental Laws or that could result in liability under any
     Environmental Law.

             "INCREASED AMOUNTS": as defined in subsection 2.15(a).

             "INDEBTEDNESS": of any Person at any date, without duplication, (a)
     all indebtedness of such Person for borrowed money or for the deferred
     purchase price of property or services (other than current trade
     liabilities incurred in the ordinary course of business and payable in
     accordance with customary practices), (b) any other indebtedness of such
     Person which is evidenced by a note, bond, debenture or similar instrument,
     (c) all obligations of such Person under Capitalized Leases, (d) all
     obligations of such Person

                                       -4-
<Page>

     in respect of acceptances issued or created for the account of such Person
     and (e) all indebtedness of others of the types described in (a) through
     (d) above secured by any Lien on any property owned by such Person even
     though such Person has not assumed or otherwise become liable for the
     payment thereof (the amount of such indebtedness with respect to such
     Person being deemed to be the lesser of the value of such property or the
     amount of indebtedness of others so secured).

             "INDENTURE": the indenture dated as of October 26, 2001 by and
     between the Borrower and the Bank of New York, as trustee, relating to the
     issuance of the Borrower's 5.5% Convertible Subordinated Notes due 2008.

             "INITIAL FINANCIAL STATEMENTS": as defined in subsection 4.1.

             "INTELLECTUAL PROPERTY": as defined in subsection 4.9.

             "INTEREST PAYMENT DATE":

                    (a)    as for any Prime Rate Loans, the last day of each
             calendar quarter, commencing with the first calendar quarter ending
             after the Closing Date;

                    (b)    as for any LIBOR Loan having an Interest Period of
             three (3) months or less, the last day of such Interest Period; and

                    (c)    as for any LIBOR Loan having an Interest Period
             longer than three months, each day which is three (3) months, or a
             whole multiple thereof, after the first day of such Interest Period
             and the last day of such Interest Period.

             "INTEREST PERIOD": with respect to any LIBOR Loan:

                    (a)    initially, the period commencing on the borrowing or
             conversion date, as the case may be, with respect to such LIBOR
             Loan and ending thirty (30), sixty (60), ninety (90) or one hundred
             eighty (180) days thereafter, as selected by the Borrower in its
             notice of borrowing or notice of conversion, as the case may be,
             given with respect thereto; and

                    (b)    thereafter, each period commencing on the last day of
             the next preceding Interest Period applicable to such LIBOR Loan
             and ending thirty (30), sixty (60), ninety (90) or one hundred
             eighty (180) days, as selected by the Borrower by irrevocable
             notice to the Lender not less than three Business Days prior to the
             last day of the then current Interest Period with respect thereto;

     PROVIDED, HOWEVER, that, all of the foregoing provisions relating to
     Interest Periods are subject to the following: (i) if any Interest Period
     pertaining to a LIBOR Loan would otherwise end on a day that is not a
     Business Day, such Interest Period shall be extended to the next succeeding
     Business Day unless the result of such extension would be to carry such
     Interest Period into another calendar month in which event such Interest
     Period shall end on the immediately preceding Business Day; (ii) the
     Borrower shall not select any Interest Period for the Loan that would
     otherwise extend beyond the Maturity Date; (iii)

                                       -5-
<Page>

     any Interest Period pertaining to a LIBOR Loan that begins on the last
     Business Day of a calendar month (or on a day for which there is no
     numerically corresponding day in the calendar month at the end of such
     Interest Period) shall end on the last Business Day of a calendar month;
     and (iv) the Borrower shall select Interest Periods so as not to require a
     payment or prepayment of any LIBOR Loan during an Interest Period for such
     LIBOR Loan.

             "INVENTORY": all "inventory" (as that term is defined in the UCC)
     of the Borrower and its Subsidiaries, and to the extent not included in
     such definition, shall also mean and include all raw materials and other
     materials and supplies, work-in-process and finished goods of the Borrower
     and its Subsidiaries and any products made or processed therefrom and all
     substances, if any, commingled therewith or added thereto.

             "INVESTMENT": as defined in subsection 6.10.

             "LATE RATE": as defined in subsection 2.10.

             "LENDER": as defined in the preamble to this Agreement.

             "LIBOR BASE RATE": with respect to each day during each Interest
     Period pertaining to a LIBOR Loan, the rate per annum equal to the average
     rate at which the Lender is offered Dollar deposits at or about 10:00 A.M.,
     Boston, Massachusetts time, three (3) Business Days prior to the beginning
     of such Interest Period by prime banks in the London interbank market where
     foreign currency and exchange operations in respect of its LIBOR Loans are
     then being conducted for delivery on the first day of such Interest Period
     for a period comparable to the number of days comprised therein and in an
     amount comparable to the amount of its LIBOR Loan to be outstanding during
     such Interest Period.

             "LIBOR LOANS": the portion of the unpaid principal amount of the
     Loan for which the applicable rate of interest is based upon the LIBOR
     Rate.

             "LIBOR RATE": with respect to each day during each Interest Period
     pertaining to a LIBOR Loan, a rate per annum determined for such day in
     accordance with the following formula (rounded upward to the nearest
     0.0005%):

                                 LIBOR BASE RATE
                        ---------------------------------
                        1.00 - LIBOR Reserve Requirements

             "LIBOR RESERVE REQUIREMENTS": for any day as applied to a LIBOR
     Loan, the aggregate (without duplication) of the rates (expressed as a
     decimal to the fourth digit) of reserve requirements in effect on such day
     (including, without limitation, basic, supplemental, marginal and emergency
     reserves under any regulations of the Board of Governors or other
     Governmental Authority having jurisdiction with respect, thereto)
     prescribed for London interbank market funding (currently referred to as
     "LIBOR Liabilities" in Regulation D of the Board of Governors) maintained
     by a member bank of the Federal Reserve System and applicable with respect
     to such LIBOR Loan.

                                       -6-
<Page>

             "LIEN": any mortgage, pledge, hypothecation, assignment, deposit
     arrangement, encumbrance, lien (statutory or other), charge or other
     security interest or any preference, priority or other security agreement
     or preferential arrangement of any kind or nature whatsoever (including,
     without limitation, any conditional sale or other title retention agreement
     and any Capitalized Lease having substantially the same economic effect as
     any of the foregoing).

             "LOAN": as defined in subsection 2.1.

             "LOAN PARTIES": the Borrower or any Subsidiary which is now or
     hereafter becomes a party to any Financing Document.

             "MATERIAL ADVERSE EFFECT" or "MATERIAL ADVERSE CHANGE": a material
     adverse effect or change on (a) the business, operations, property or
     condition (financial or otherwise) of the Borrower and its Subsidiaries
     taken as a whole or (b) the validity or enforceability of this Agreement or
     any of the other Financing Documents or the rights or remedies of the
     Lender hereunder or thereunder.

             "MATURITY DATE": June 30, 2005.

             "NET PROCEEDS": the aggregate cash proceeds received by the
     Borrower or any Subsidiary (including any cash payments received by way of
     deferred payment of principal pursuant to a note or installment receivable
     or purchase price adjustment receivable or otherwise, but only as and when
     received) in respect of any Asset Sale (including any proceeds of insurance
     received upon any casualty or loss); in each case net of (without
     duplication), (i) the amount required to repay any Indebtedness (other than
     the Loans) secured by a Lien on any assets of the Borrower or its
     Subsidiaries that are collateral for any such debt securities or loans that
     are sold or otherwise disposed of in connection with such Asset Sale, (ii)
     the reasonable expenses (including legal fees and brokers' and
     underwriters' commissions, lenders' fees or credit enhancement fees, in any
     case, paid to third parties or, to the extent permitted hereby, Affiliates)
     incurred in effecting such issuance or sale and (iii) any taxes reasonably
     attributable to such sale and reasonably estimated by the Borrower or its
     Subsidiaries to be actually payable.

             "NON-EXCLUDED TAXES": as defined in subsection 2.15(a).

             "NOTE": as defined in subsection 2.1(b).

             "OBLIGATIONS": all Indebtedness, obligations and liabilities of the
     Borrower or any and all of its Subsidiaries to the Lender, individually or
     collectively, now existing or hereafter arising, direct or indirect, joint
     or several, absolute or contingent, matured or unmatured, liquidated or
     unliquidated, secured or unsecured, arising by contract, operation of law
     or otherwise, arising or incurred under this Agreement or any of the other
     Financing Documents or in respect of the Loan or the Note or other
     instruments at any time evidencing any thereof.

                                       -7-
<Page>

             "OBSOLETE PROPERTY": any property of the Borrower or any of its
     Subsidiaries which is obsolete, outdated or worn out or the useful life of
     which has ended, in each case in the good faith determination of the
     Borrower or any applicable Subsidiary.

             "PBGC": the Pension Benefit Guaranty Corporation established
     pursuant to Subtitle A of Title IV of ERISA and any Governmental Authority
     which succeeds to the powers and functions thereof.

             "PERMITTED LIENS": as defined in subsection 6.4.

             "PERSON": an individual, partnership, corporation, business trust,
     joint stock company, limited liability company, trust, unincorporated
     association, joint venture, Governmental Authority or other entity of
     whatever nature.

             "PLAN": at a particular time, any employee benefit plan which is
     covered by ERISA and in respect of which the Borrower or a Commonly
     Controlled Entity is (or, if such plan were terminated at such time, would
     under Section 4069 of ERISA be deemed to be) an "employer" as defined in
     Section 3(5) of ERISA.

             "PRIME RATE": for any day, a rate equal to the variable rate of
     interest per annum, most recently announced by the Lender at its
     headquarters in Boston, Massachusetts, as its "prime rate," with the
     understanding that the Lender's "prime rate" is one of its interest rates
     and serves as a basis upon which effective rates of interest are calculated
     for loans making reference thereto and may not be the lowest of the
     Lender's interest rates. Any change in the Prime Rate shall be effective as
     of the effective date stated in the announcement by the Lender of such
     change.

             "PRIME RATE LOANS": the portion of the unpaid principal amount of
     the Loan for which the rate of interest is based upon the Prime Rate.

             "PURCHASE MONEY INDEBTEDNESS": any Indebtedness incurred by the
     Borrower or any of its Subsidiaries, whichever is applicable, in connection
     with the acquisition by the Borrower or any of its Subsidiaries, whichever
     is applicable, of any real or personal property.

             "REPORTABLE EVENT": any of the events set forth in Section 4043(b)
     of ERISA, other than those events as to which the thirty day notice period
     is waived under subsections .13, .14, .16, .18, .19 or .20 of PBGC Reg.
     Section 2615.

          "REQUIREMENT OF LAW": as to any Person, any law, treaty, rule or
     regulation or determination of an arbitrator or a court or other
     Governmental Authority, in each case applicable to or binding upon such
     Person or any of its property or to which such Person or any of its
     property is subject.

          "RESPONSIBLE OFFICER": as to any Person, the chief executive officer
     and the president of such Person or, with respect to financial matters, the
     chief financial officer of such Person or, in either case, such other
     executive officers as may be designated from time to time by such Person in
     writing to the Lender.

                                       -8-
<Page>

             "RESTRICTED CASH": as defined in accordance with GAAP.

             "RESTRICTED PAYMENTS": as defined in subsection 6.9.

             "SEC": the United States Securities and Exchange Commission or any
     other federal governmental agency which may hereafter perform its
     functions.

             "SECURITY AGREEMENTS": collectively, the Borrower Security
     Agreement and the Subsidiary Security Agreements.

             "SECURITY DOCUMENTS": collectively, the Security Agreements and all
     other security agreements, pledge agreements, financing statements,
     assignments, mortgages, agreements, documents and instruments now or
     hereafter delivered to the Lender granting a Lien on any asset or assets of
     any Person to secure the Obligations or to secure any guarantee of any such
     Obligations and, including, without limitation, any such document delivered
     pursuant to subsections 5.10 and 5.11.

             "SOLVENT": when used with respect to any Person, means that, as of
     any date of determination, (a) the amount of the "present fair saleable
     value" of the assets of such Person will, as of such date, exceed the
     amount that will be required to pay all "liabilities of such Person,
     contingent or otherwise", as of such date (as such quoted terms are
     determined in accordance with applicable federal and state laws governing
     determinations of the insolvency of debtors) as such debts become absolute
     and matured, (b) such Person will not have, as of such date, an
     unreasonably small amount of capital with which to conduct its business,
     and (c) such Person will be able to pay its debts as they mature, taking
     into account the timing of and amounts of cash to be received by such
     Person and the timing of and amounts of cash to be payable on or in respect
     of indebtedness of such Person; in each case after giving effect to (A) as
     of the Closing Date the making of the extensions of credit to be made on
     the Closing Date and to the application of the proceeds of such extensions
     of credit and (B) on any date after the Closing Date, the making of any
     extension of credit to be made on such date, and to the application of the
     proceeds of such extension of credit. For purposes of this definition, (i)
     "debt" means liability on a "claim", and (ii) "claim" means any (x) right
     to payment, whether or not such a right is reduced to judgment, liquidated,
     unliquidated, fixed, contingent, matured, unmatured, disputed, undisputed,
     legal equitable, secured or unsecured or (y) right to an equitable remedy
     for breach of performance if such breach gives rise to a right to payment,
     whether or not such right to an equitable remedy is reduced to judgment,
     fixed, contingent, matured or unmatured, disputed, undisputed, secured or
     unsecured.

             "SUBORDINATED DEBT": any and all Indebtedness, liabilities and
     obligations of the Borrower to any Person (other than the Lender) which is
     subordinated to the Obligations upon terms and conditions which are
     reasonably satisfactory to the Lender and shall include Indebtedness
     pursuant to the Indenture.

                                       -9-
<Page>

             "SUBORDINATION AGREEMENTS": any subordination agreement (or
     provision) by which the Subordinated Debt is subordinated to the
     Obligations upon terms and conditions which are reasonably satisfactory to
     the Lender.

             "SUBSIDIARY": as to any Person, a corporation, partnership or other
     entity of which shares of stock or other ownership interests having
     ordinary voting power (other than stock or such other ownership interests
     having such power only by reason of the happening of a contingency) to
     elect a majority of the board of directors or other managers of such
     corporation, partnership or other entity are at the time owned, or the
     management of which is otherwise controlled, directly or indirectly through
     one or more intermediaries, or both, by such Person. Unless otherwise
     qualified, all references to a "Subsidiary" or to "Subsidiaries" in this
     Agreement shall refer to a Subsidiary or Subsidiaries of the Borrower.

             "SUBSIDIARY GUARANTY": the Subsidiary Guaranty to be executed and
     delivered by each Domestic Subsidiary, substantially in the form of EXHIBIT
     C as the same may be amended, supplemented or otherwise modified from time
     to time.

             "SUBSIDIARY SECURITY AGREEMENT": the Subsidiary Security Agreement
     - All Assets to be executed and delivered by each Domestic Subsidiary in
     favor of the Lender, substantially in the form of EXHIBIT D, as the same
     may be amended, supplemented or otherwise modified from time to time.

             "TAX CODE": the Internal Revenue Code of 1986, as amended from time
     to time.

             "TYPE": as to any Loan, its nature as a Prime Rate Loan or a LIBOR
     Loan.

             "UCC": the Uniform Commercial Code as from time to time in effect
     in The Commonwealth of Massachusetts.

             "UNRESTRICTED CASH": any cash that is not Restricted Cash.

     2.      USE OF TERMS. The use of the singular of terms which are defined in
the plural shall mean and refer to any one of them; and pronouns used herein
shall be deemed to include the singular and the plural and all genders. The use
of the connective "or" is not intended to be exclusive; the term "may not" is
intended to be prohibitive and not permissive; use of "includes" and "including"
is intended to be interpreted as expansive and amplifying and not as limiting in
any way.

     The words "hereof", "herein" and "hereunder" and words of similar import
when used in this Agreement shall refer to this Agreement as a whole and not to
any particular provision of this Agreement. All references in this Agreement to
Articles, Sections, Exhibits, Schedules and Appendices refer to Articles,
Sections, Exhibits, Schedules and Appendices of this Agreement unless otherwise
indicated. All Exhibits, Schedules and Appendices attached to this Agreement are
incorporated herein and made a part hereof.

                                      -10-
<Page>

--------------------------------------------------------------------------------

                                CREDIT AGREEMENT
                                (the "AGREEMENT")

                                 by and between

                         CITIZENS BANK OF MASSACHUSETTS
                                 (the "LENDER")

                                       and

                          CUBIST PHARMACEUTICALS, INC.
                                (the "BORROWER")

--------------------------------------------------------------------------------

                                  LIEN SEARCHES

                                   APPENDIX B

<Page>

                                                                      APPENDIX C

                        [FORM OF COMPLIANCE CERTIFICATE]

                             COMPLIANCE CERTIFICATE

     Reference is hereby made to a certain Credit Agreement, dated as of
February __, 2003 (as the same may be amended, modified, supplemented, extended
or restated, from time to time, the "CREDIT AGREEMENT") by and between CUBIST
PHARMACEUTICALS, INC., a Delaware corporation (the "BORROWER") and CITIZENS BANK
OF MASSACHUSETTS, a Massachusetts bank (the "LENDER"). All capitalized terms not
defined herein but defined in the Credit Agreement shall have the meanings given
to such terms in the Credit Agreement.

     The undersigned hereby certifies that he or she is a Responsible Officer of
the Borrower and as such, is authorized, for and on behalf of the Borrower, to
execute and deliver this Compliance Certificate to the Lender in accordance with
the provisions of the Credit Agreement. Pursuant to the provisions of subsection
5.1(c) of the Credit Agreement, the undersigned hereby certifies to the Lender
as follows:

             1.     Each of the representations and warranties made by the
     Borrower and its Subsidiaries in or pursuant to the Financing Documents are
     true and correct in all material respects on and as of the date hereof, as
     if made on and as of the date hereof, except (a) to the extent such
     representations and warranties expressly relate to an earlier date in which
     case such representations and warranties shall be true and correct in all
     material respects as of such earlier date, and (b) as follows:

                    [Describe divergences, if any]

             2.     Since the end of the last fiscal quarter of the Borrower, no
     Material Adverse Change has occurred except:

                    [Describe, if any]

             3.     Except as set forth in the certificates attached hereto and
     except as heretofore disclosed to the Lender in previous Compliance
     Certificates, there has been no change (i) in the Certificate of
     Incorporation or By-laws of the Borrower, or (ii) in the incumbency of the
     officers of the Borrower whose signatures have heretofore been certified to
     the Lender.

             4.     The financial statements submitted herewith (if any) are in
     compliance with the applicable provisions of subsections 5.1 and 5.2 of the
     Credit Agreement.

             5.     The undersigned has reviewed or caused to be reviewed all of
     the Financing Documents, and based upon such review and to the knowledge of
     the undersigned, no Default or Event of Default has occurred and is
     continuing as of the date hereof (or if applicable, will occur after giving
     effect to the making of the Loans requested to be made on the date hereof),
     except as follows:

<Page>

                    [Describe Defaults or Events of Default]

             6.     Attached hereto as EXHIBIT 1 are calculations demonstrating
     that, based upon the financial statements of the Borrower and its
     Subsidiaries submitted herewith (if any), the Borrower and its Subsidiaries
     were in compliance as of the date of such financial statements with all
     financial covenants set forth in subsection 5.13 of the Credit Agreement to
     be measured as of such date, except as noted on EXHIBIT 1 attached hereto.

             7.     Any changes in the chief executive office and chief place of
     business of the Borrower or any of its Subsidiaries which have occurred
     and/or any additional locations at which any of the Inventory or equipment
     are kept, notice of which has not yet been provided to the Lender, in
     accordance with the provisions of the Security Documents, are set forth
     below:

                    [Describe]

     EXECUTED under seal as of this ________ day of __________________, ______.

                                              CUBIST PHARMACEUTICALS, INC.

                                              By:
                                                 --------------------------
                                                 Name:
                                                 Title:
                                                   Its duly authorized officer

                                        2
<Page>

                                                                       EXHIBIT 1

     SECTION 5.13 INVESTMENTS

     (a)  Actual Unrestricted Cash and Cash Equivalents:
                                                                 --------------
          Required Unrestricted Cash and Cash Equivalents:
                                                                 --------------

     (b)  1. Unrestricted Cash & Cash Equivalents:
                                                                 --------------
          2. Total Liabilities:
                                                                 --------------
             Minus

          3. Subordinated Debt:
                                                                 --------------
             Equals

                                                                 --------------
          4. Defined Total Liabilities (2 Minus 3):
                                                                 --------------

          Ratio (1 divided by 4):
                                                                 --------------
          Required Ratio:                                          1.00
                                                                 --------------

     SECTION 6.15 CURRENT MATURITIES OF LONG TERM DEBT

          1. Current Maturities of Long Term Debt:
                                                                 ---------------
          2. Principal Payments Last 12 Months:
                                                                 ---------------

             Greater of (1) or (2):
                                                                 ---------------
             vs.
             Maximum Permitted:                                    6,000,000
                                                                 ---------------

                                        3

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