Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2012

Summary of Significant Accounting Policies

Schedule of non-controlling interest in subsidiary

(in thousands)	Non-Controlling Interest
Balance at December 31, 2010	$	1,027
Net loss attributable to Silver Creek	(223		)
Balance at June 30, 2011	$	804

	Non-Controlling Interest
Balance at December 31, 2011	$	574
Net loss attributable to Silver Creek	(231		)
Balance at June 30, 2012	$	343

Schedule of available-for-sale securities

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
	(in thousands)
June 30, 2012:
U.S. government agencies securities	$	1,000	$	—	$	(1	)	$	999
Corporate debt securities	73,718		1		(49		)	73,670
Total	$	74,718	$	1	$	(50	)	$	74,669

Schedule of marketable securities in an unrealized loss position

	Aggregate Fair Value		Unrealized Losses
	(in thousands)
June 30, 2012:
U.S. government agencies securities	$	999	$	(1	)
Corporate debt securities	65,172		(49		)
Total	$	66,171	$	(50	)

Schedule of comprehensive loss from operations

	Three Months Ended June 30,						Six Months Ended June 30,
(in thousands)	2011			2012			2011			2012
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(29,051	)	$	(20,026	)	$	(42,508	)	$	(43,310	)
Unrealized loss on available-for-sale securities	—			(49		)	—			(49		)
Comprehensive loss	$	(29,051	)	$	(20,075	)	$	(42,508	)	$	(43,359	)