Exhibit 10.26  AMENDMENT
NO. 4 to the MANUFACTURING AND SUPPLY AGREEMENT entered into as of September 30, 2001 by and between ACS Dobfar, SpA and Cubist Pharmaceuticals, Inc.  This
AMENDMENT NO. 4(Amendment No. 4), to that certain Manufacturing and Supply
Agreement (as amended to date pursuant to Amendments Nos. 1, 2 and 3, the Agreement)
entered into as of September 30, 2001, is made this 22nd day of September,
2006 (Amendment No. 4 Effective Date), by and between ACS Dobfar, SpA,
an Italian corporation (ACSD), and Cubist Pharmaceuticals, Inc.,
a Delaware corporation (Cubist). Unless otherwise defined herein,
capitalized terms used but not defined herein shall have the meaning set forth
in the Agreement, and the Agreement shall be amended to incorporate any
additional definitions provided for in this Amendment No. 4, including
definitions in the preamble and recitals hereto.  WHEREAS,
Cubist and ACSD desire to amend the Agreement as set forth in this Amendment No. 4;  NOW, THEREFORE,
in consideration of the foregoing and the mutual promises made herein, and for
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree to amend the Agreement as
follows:  1.             Amendment of Section 1.  The Agreement is hereby amended by deleting
the definitions of CIP and Contract Year.  2.             Amendment
of Section 5.1.  The
Agreement is hereby amended by deleting the table included in Section 5.1
of the Agreement and by replacing the following yearly purchases: at the end
of the first paragraph of Section 5.1 with the following:  minimum
purchases in the amount of one thousand kilograms (1,000 kg) in each calendar
year until the end of the term of this Agreement in 2015.  3.Amendment of Section 5.7.The Agreement is
hereby amended by:  (a)           Replacing Payment
Schedule in the heading of Section 5.7 of the Agreement with Invoicing; and  (b)           deleting the remainder of Section 5.7,
starting with  will follow promptly behind the invoicing and replacing such
language with the following:  , as well as any other Product supplied under
this Agreement, shall be in accordance with Section 6.3(a).  4.             Amendment
of Section 6.1.  The
Agreement is hereby amended by inserting the word base in between at a and price
in the first paragraph of Section 6.1 of the Agreement and by adding the
following language at the end of such first paragraph, immediately preceding
the period:  through [ ]* and [ ]* in [ ]* ( the Base Price), and the Base Price
shall [ ]*. [ ]*  Notwithstanding
anything in the foregoing to the contrary, from [ ]* until [ ]*, Cubist shall
pay [ ]* for every [ ]* hereunder.  On or
around [ ]* after the Amendment No. 4  
 *Confidential
Treatment Requested.  Omitted portions
filed with the Securities and Exchange Commission (the Commission).


Effective
Date, the parties will discuss in good faith whether any adjustment to the
above is appropriate.  In addition, if
the [ ]* above or below [ ]* on or after [ ]*, the parties shall discuss in good
faith whether an adjustment to the Base Price is appropriate.  The
second paragraph of Section 6.1 is hereby amended by inserting the words the
Base Price of in between will pay and [ ]* in the second-to-last line of
such second paragraph and by adding the following language at the end of such
second paragraph, immediately preceding the period:  ,
subject to the first paragraph of this Section 6.1.  5.             Amendment
of Section 6.3(a). 
The Agreement is hereby amended by deleting receipt by Cubist of in
the first line of Section 6.3(a) of the Agreement and by replacing Certificate
of Analysis, Batch Records, and Manufacturing Compliance Reports as required by
Exhibit D with respect to such Product. in the first sentence of such Section with
provided  that,
Cubist shall not have an obligation to pay an invoice [ ]*.  6.             Amendment
of Section 6.4. The Agreement is hereby amended by deleting
the first three sentences of Section 6.4 of the Agreement and the
following language at the beginning of the fourth paragraph: Notwithstanding
the above, for calendar year 2005 only, and by replacing both instances of the
word Milan with Agnani, such that Section 6.4 shall begin The Product
will be delivered Ex works Agnani,   7.             Amendment
of Section 12.1. The Agreement is hereby amended by
deleting Section 12.1 thereof in its entirety and replacing it with the
following Section 12.1:  12.1        Term  This
Agreement shall take effect as of the Effective Date and shall remain in effect
until December 31, 2015, unless sooner terminated in accordance with Section 12.2
or extended in accordance with this Section 12.1. Unless, not later than [
]*, Cubist notifies ACSD in writing that Cubist does not desire to extend the
term of Agreement, the term of the Agreement will be extended for an additional
two years; provided, that, ACSD and Cubist have
negotiated in good faith a revision to the prices charged for Product, pursuant
to Section 6.1 hereof, based on ACSDs then current costs to manufacture
Product. All applicable terms and conditions of this Agreement shall remain in
effect during the extension term, unless expressly amended in writing by the
parties. Cubist may renew this Agreement for additional two-year extension
terms by  providing written renewal notice to ACSD not
later than twelve (12) months prior to the expiration of the then current
extension term.  8.             Amendment
to Exhibits.  The
Agreement is hereby amended by replacing Exhibits C-1 (Master Product
Specifications) and D (Quality Requirements) with the attached Exhibits C and
D.  The parties intend to amend Exhibit D
in the near future.  9.             No Other
Amendments.Except to the
extent amended hereby, all of the definitions, terms, provisions and conditions
set forth in the Agreement are hereby ratified and confirmed and shall remain
in full force and effect.  The terms and
conditions herein and subject matter hereof shall at all times be  
 *
Confidential Treatment Requested. 
Omitted portions filed with the Commission.  2


considered
Confidential Information of Cubist, as defined in the Agreement. The Agreement
and this Amendment No. 4 shall be read and construed together as a single
agreement and the term Agreement  
 *
Confidential Treatment Requested.  Omitted
portions filed with the Commission.  3


shall be deemed a
reference to the Agreement as amended by this Amendment No. 4.  This Amendment No. 4 may be signed in
any number of counterparts, each of which shall be deemed to be an original and
all of which together shall constitute but one and the same instrument.  In making proof of this Amendment No. 4
it shall not be necessary to produce or account for more than one such
counterpart.  IN WITNESS WHEREOF, this Amendment No. 4 has been executed under seal by the parties
hereto to be effective as of the day and year first above written.  ACS DOBFAR SpABy:
  /s/ Marco FalcianiName:Marco FalcianiTitle: PresidentDate: 9/22/06CUBIST PHARMACEUTICALS, INC.By:
  /s/ Michael W. BonneyName: Michael W. BonneyTitle: President & CEODate: 9/22/06