Exhibit 5.1 [LETTERHEAD OF KRAMER LEVIN NAFTALIS & FRANKEL LLP] November 24, 2004 SIGA Technologies, Inc. 420 Lexington Avenue Suite 601 New York, New York 10170 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 1,000,000 shares (the "Shares") of common stock, par value $0.0001 per share, of the Company (the "Common Stock"). 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 Shares have been legally issued, and are 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 Company. Very truly yours, /s/ KRAMER LEVIN NAFTALIS & FRANKEL LLP