Ownership Submission
FORM 3
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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(Print or Type Responses)
1. Name and Address of Reporting Person *
  PORTER MICHAEL E
2. Date of Event Requiring Statement (Month/Day/Year)
01/31/2012
3. Issuer Name and Ticker or Trading Symbol
MERRIMACK PHARMACEUTICALS INC [MACK]
(Last)
(First)
(Middle)
C/O MERRIMACK PHARMACEUTICALS, INC., ONE KENDALL SQUARE, SUITE B7201
4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
__X__ Director _____ 10% Owner
_____ Officer (give title below) _____ Other (specify below)
5. If Amendment, Date Original Filed(Month/Day/Year)
(Street)

CAMBRIDGE, MA 02139
6. Individual or Joint/Group Filing(Check Applicable Line)
_X_ Form filed by One Reporting Person
___ Form filed by More than One Reporting Person
(City)
(State)
(Zip)
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)
Common Stock 63,000
D
 

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly. SEC 1473 (7-02)
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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 of Derivative Security: 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
Series C Convertible Preferred Stock   (1)   (1) Common Stock 56,509 $ (1) D  
Series D Convertible Preferred Stock   (2)   (2) Common Stock 34,286 $ (2) D  
Series E Convertible Preferred Stock   (3)   (3) Common Stock 25,000 $ (3) D  
Series F Convertible Preferred Stock   (4)   (4) Common Stock 33,000 $ (4) D  
Series G Convertible Preferred Stock   (5)   (5) Common Stock 28,570 $ (5) D  
Stock Option (right to buy)   (6) 05/08/2013 Common Stock 15,000 $ 2.19 D  
Stock Option (right to buy)   (6) 08/03/2015 Common Stock 10,000 $ 1.71 D  
Stock Option (right to buy)   (6) 10/04/2017 Common Stock 2,500 $ 2.59 D  
Stock Option (right to buy)   (6) 09/21/2018 Common Stock 2,500 $ 1.81 D  
Stock Option (right to buy)   (7) 11/04/2019 Common Stock 5,000 $ 2.12 D  
Stock Option (right to buy)   (8) 10/14/2020 Common Stock 25,000 $ 2.69 D  
Stock Option (right to buy)   (6) 12/08/2020 Common Stock 60,000 $ 2.69 D  
Stock Option (right to buy)   (6) 05/02/2021 Common Stock 25,000 $ 5.54 D  

Reporting Owners

Reporting Owner Name / Address Relationships
Director 10% Owner Officer Other
PORTER MICHAEL E
C/O MERRIMACK PHARMACEUTICALS, INC.
ONE KENDALL SQUARE, SUITE B7201
CAMBRIDGE, MA 02139
  X      

Signatures

/s/ Jeffrey A. Munsie, attorney-in-fact 02/01/2012
**Signature of Reporting Person Date

Explanation of Responses:

* 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).
(1) Each share of Series C Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(2) Each share of Series D Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(3) Each share of Series E Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(4) Each share of Series F Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(5) Each share of Series G Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(6) This option is fully vested.
(7) This option vested as to 1/12th of the shares on each of November 5, 2009 and February 1, 2010 and vests in equal quarterly installments thereafter until August 1, 2012.
(8) This option vested as to 1/12th of the shares on each of October 15, 2010 and January 1, 2011 and vests in equal quarterly installments thereafter until July 1, 2013.

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