Exhibit 99.1

  Merrimack Pharmaceuticals Secures $9 Million Credit Facility from
               Hercules Technology Growth Capital, Inc.

    CAMBRIDGE, Mass.--(BUSINESS WIRE)--June 20, 2005--

            Merrimack Pharmaceuticals Closes on $37 Million
                 of Series D Equity and Loan Financing

    Merrimack Pharmaceuticals, Inc. announced on April 20, 2005 that
it has completed equity and debt financings totaling $37.3 million.
The financing was comprised of $28.3 million of Series D Preferred
Stock and a $9 million venture loan.
    Returning equity investors include Sorenson Development, Inc. and
affiliates, Unilever Technology Ventures Fund B.V., and Wharton
Biotechnology Partners. New equity investors include, among others, WT
Investment Advisors. The venture loan was provided by Palo Alto,
California-based Hercules Technology Growth Capital (NASDAQ:HTGC).
Since inception, Merrimack has raised approximately $78 million of
equity capital, and $87 million of total capital.
    "This financing provides the Company with the resources to advance
our pipeline and research activities through several key milestones,"
said Robert Mulroy, President and CEO of Merrimack. "We are pleased by
the strong and continuing support from our investors."
    Merrimack Pharmaceuticals, Inc. is a biotechnology company focused
on the discovery and development of novel treatments for diseases in
the areas of autoimmune disease and cancer. Its lead compound, MM-093
is currently in clinical development to treat patients with rheumatoid
arthritis. MM-093 is an investigational drug and has not been approved
by the U.S. Food and Drug Administration or any international
regulatory agency. The company's proprietary Network Biology discovery
platform, developed with the help of leading scientists from MIT and
Harvard, enables the high throughput profiling of protein networks as
a basis for improved validation, lead identification and speed in the
development of innovative, effective and safe therapeutics.

    About Merrimack Pharmaceuticals:

    Merrimack is a privately held company based in Cambridge,
Massachusetts. For additional information, please visit
http://www.merrimackpharma.com.

    About Hercules Technology Growth Capital:

    Founded in December, 2003, Hercules Technology Growth Capital, Inc
(NASDAQ:HTGC) is a publicly traded specialized finance company
providing debt and equity growth capital to technology-related
companies at all stages of development. The Company primarily finances
privately-held companies backed by leading venture capital and private
equity firms and also may finance certain publicly-traded companies.
Hercules focuses its investments in companies active in technology
industry sub-sectors, characterized by products or services that
require advanced technologies, including computer software and
hardware, networking systems, semiconductors, semiconductor capital
equipment, information technology infrastructure or services, Internet
consumer and business services, telecommunications, telecommunications
equipment, media and life sciences. The Company's investments are
originated through its principal office located in Silicon Valley, as
well as additional offices in the Boston and Chicago areas. Providing
capital to privately-held companies backed by leading venture capital
and private equity firms involves a certain degree of credit risk and
may result in potential losses. For more information or companies
interested in learning more about financing opportunities should
contact info@herculestech.com or call at 650-289-3060 or visit
http://www.herculestech.com.

    CONTACT: Hercules Technology Growth Capital, Inc.
             Main, 650-289-3060
             info@herculestech.com
             Scott Harvey, 650-289-3078 (Chief Legal Officer)