May 4, 2000

Siga Technologies, Inc.
420 Lexington Avenue - Suite 620
New York, New York 10170


           Re:  Siga Technologies, Inc.
                Registration Statement on Form S-3

           We have  acted as  counsel  to Siga  Technologies,  Inc.,  a Delaware
corporation (the "Company"),  in connection with the preparation and filing of a
Registration  Statement  on Form S-3  (the  "Registration  Statement")  with the
Securities and Exchange  Commission under the Securities Act of 1933, as amended
(the "Act").  The Registration  Statement relates to the sale by certain selling
stockholders  of the Company of up to 2,833,391  shares of the Company's  Common
Stock (the  "Shares").  This opinion is being  furnished in accordance  with the
requirements of item 16 of Form S-3 and Item 601(b)(5)(i) of Regulation S-K.

           We have  reviewed the Company's  charter  documents and the corporate
proceedings  taken by the Company in connection with the issuance of the Shares.
Based on such  review,  we are of the  opinion  that the  Shares  have been duly
authorized,  and if,  as and  when  sold in  accordance  with  the  Registration
Statement and the related  prospectus (as amended and  supplemented  through the
date of sale) will be legally issued, fully paid and nonassessable.

           We consent to the filing of this opinion letter as Exhibit 5.1 to the
Registration  Statement  and to the  reference  to this firm  under the  caption
"Legal Matters" in the prospectus which is part of 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 Act, the rules and
regulations of the Securities and Exchange Commission promulgated thereunder, or
Item 509 of Regulation S-K. This opinion letter is rendered as of the date first
written  above  and  we  disclaim  any   obligation  to  advise  you  of  facts,
circumstances,  events or  developments  which  hereafter  may be brought to our
attention and which may alter,  affect or modify the opinion  expressed  herein.
Our opinion is expressly limited to the matters set forth above and we render no
opinion,  whether by implication or otherwise,  as to any other matters relating
to the Company or the Shares.

                                Very truly yours,

                               /s/ Orrick, Herrington & Sutcliffe LLP