Exhibit 5.1

KRAMER LEVIN NAFTALIS & FRANKEL LLP

                                                        March 31, 2006

SIGA Technologies, Inc.
420 Lexington Avenue, Suite 408
New York, New York 10170

            Re:   SIGA Technologies, Inc.

Dear Ladies and Gentlemen:

      We have acted as counsel to SIGA Technologies, Inc., a Delaware
corporation (the "Registrant"), in connection with the preparation and filing of
a Registration Statement on Form S-3 (the "Registration Statement") with the
Securities and Exchange Commission (the "Commission"), with respect to the
registration under the Securities Act of 1933, as amended (the "Act"), of an
aggregate of 2,000,000 shares of common stock, par value $0.0001 per share, of
the Registrant (the "Common Stock"), issuable upon the exercise of certain
additional investment rights.

      We have reviewed copies of the Registration Statement, the Restated
Certificate of Incorporation of the Registrant, the Bylaws of the Registrant, as
amended, and resolutions of the Board of Directors of the Registrant.

            We have also reviewed such other documents and made such other
investigations as we have deemed appropriate. As to various questions of fact
material to this opinion, we have relied upon statements, representations and
certificates of officers or representatives of the Registrant, public official
and others. We have not independently verified the facts so relied on.

      Based upon the foregoing, and subject to the qualifications, limitations
and assumptions set forth herein, we are of the opinion that the Common Stock,
when issued and paid for in accordance with the terms and conditions of the
respective additional investment rights governing such issuance, will be legally
issued, fully paid and non-assessable.

            We hereby consent to the use of this opinion as an exhibit to the
Registration Statement and to the use of our name under the caption "Legal
Matters" in the prospectus included in the Registration Statement. In giving
this consent, we do not thereby admit that we are within the category of persons
whose consent is required under Section 7 of the Securities Act or the rules and
regulations promulgated thereunder.

            We do not express any opinion with respect to any law other than the
Delaware General Corporation Law and the federal laws of the United States of
America. This opinion is rendered only with respect to the laws and legal
interpretations and the facts and circumstances in effect on the date hereof.

            Thomas E. Constance, a member of this Firm, is also a director of
the Registrant.

                                       Very truly yours,


                                       /s/ KRAMER LEVIN NAFTALIS & FRANKEL LLP

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