Washington, D.C. 20549


Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940
OMB Number: 3235-0104
Estimated average burden
hours per response: 0.5
1. Name and Address of Reporting Person*
McClements William M.

(Last) (First) (Middle)


(City) (State) (Zip)
2. Date of Event Requiring Statement (Month/Day/Year)
3. Issuer Name and Ticker or Trading Symbol
4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
Director 10% Owner
X Officer (give title below) Other (specify below)
SVP of Corporate Operations
5. If Amendment, Date of Original Filed (Month/Day/Year)
6. Individual or Joint/Group Filing (Check Applicable Line)
X Form filed by One Reporting Person
Form filed by More than One Reporting Person
Table I - Non-Derivative Securities Beneficially Owned
1. Title of Security (Instr. 4) 2. Amount of Securities Beneficially Owned (Instr. 4) 3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5) 4. Nature of Indirect Beneficial Ownership (Instr. 5)
Table II - Derivative Securities Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4) 2. Date Exercisable and Expiration Date (Month/Day/Year) 3. Title and Amount of Securities Underlying Derivative Security (Instr. 4) 4. Conversion or Exercise Price of Derivative Security 5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5) 6. Nature of Indirect Beneficial Ownership (Instr. 5)
Date Exercisable Expiration Date Title Amount or Number of Shares
Stock Option (right to buy) (1) 11/01/2021 Common Stock 150,000 6.78 D
Explanation of Responses:
1. This option vests as to 1/6th of the shares on March 19, 2012 and vests in equal quarterly installments thereafter until September 19, 2014.
/s/ William M. McClements 02/01/2012
** Signature of Reporting Person Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 5 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.

       Know all by these presents, that the undersigned hereby
makes, constitutes and appoints each of William A. Sullivan,
Jeffrey A. Munsie, Ian R. Kaminski and Christopher J.
Frankenfield, signing singly and each acting individually, as the
undersigned's true and lawful attorney-in-fact with full power
and authority as hereinafter described to:
       (1)        execute for and on behalf of the undersigned, in
the undersigned's capacity as an Officer and a Director of
Merrimack Pharmaceuticals, Inc. (the "Company"), Forms 3, 4, and
5 (including any amendments thereto) in accordance with
Section 16(a) of the Securities Exchange Act of 1934 and the
rules thereunder (the "Exchange Act");
       (2)        do and perform any and all acts for and on
behalf of the undersigned which may be necessary or desirable to
prepare, complete and execute any such Form 3, 4, or 5, prepare,
complete and execute any amendment or amendments thereto, and
timely deliver and file such form with the United States
Securities and Exchange Commission (the "SEC") and any stock
exchange or similar authority, including without limitation the
filing of a Form ID or any other application materials to enable
the undersigned to gain or maintain access to the Electronic Data
Gathering, Analysis and Retrieval system of the SEC;
       (3)        seek or obtain, as the undersigned's
representative and on the undersigned's behalf, information
regarding transactions in the Company's securities from any third
party, including brokers, employee benefit plan administrators
and trustees, and the undersigned hereby authorizes any such
third party to release any such information to the herein
appointed attorney-in-fact and approves and ratifies any such
release of information; and
       (4)        take any other action of any type whatsoever in
connection with the foregoing which, in the opinion of such
attorney-in-fact, may be of benefit to, in the best interest of,
or legally required by, the undersigned, it being understood that
the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such
form and shall contain such terms and conditions as such
attorney-in-fact may approve in such attorney-in-fact's
       The undersigned hereby grants to each such attorney-in-fact
full power and authority to do and perform any and every act and
thing whatsoever requisite, necessary, or proper to be done in
the exercise of any of the rights and powers herein granted, as
fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution
or revocation, hereby ratifying and confirming all that such
attorney-in-fact, or such attorney-in-fact's substitute or
substitutes, shall lawfully do or cause to be done by virtue of
this power of attorney and the rights and powers herein granted.
The undersigned acknowledges that the foregoing
attorneys-in-fact, in serving in such capacity at the request of
the undersigned, are not assuming nor relieving, nor is the
Company assuming nor relieving, any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.
The undersigned acknowledges that neither the Company nor the
foregoing attorneys-in-fact assume (i) any liability for the
undersigned's responsibility to comply with the requirement of
the Exchange Act, (ii) any liability of the undersigned for any
failure to comply with such requirements, or (iii) any obligation
or liability of the undersigned for profit disgorgement under
Section 16(b) of the Exchange Act.
       This Power of Attorney shall remain in full force and
effect until the undersigned is no longer required to file
Forms 3, 4, and 5 with the SEC with respect to the undersigned's
holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing
delivered to the foregoing attorneys-in-fact.
       IN WITNESS WHEREOF, the undersigned has caused this Power
of Attorney to be executed as of this 15th day of December,

/s/ William M. McClements

William M. McClements