EXHIBIT 5.1


         OPINION AND CONSENT OF ALLEN MATKINS LECK GAMBLE & MALLORY LLP

                                                               February 13, 2004

Acacia Research Corporation
500 Newport Center Drive
Newport Beach, California 92660

       Re: Acacia Research Corporation Registration Statement on Form S-3

Ladies and Gentlemen:

         We have examined the Registration Statement on Form S-3 to be filed by
Acacia Research Corporation (the "Company") with the Securities and Exchange
Commission on or about February 13, 2004 (the "Registration Statement") in
connection with the registration under the Securities Act of 1933, as amended
(the "Securities Act"), for offering and sale by the Company from time to time
of up to $50,000,000 aggregate initial offering price of the "Securities" (as
listed below):

         o    the Company's Acacia Research - CombiMatrix Common Stock;

         o    the Company's Acacia Research - Acacia Technologies Common Stock;
              and

         o    warrants to purchase the Company's Acacia Research - CombiMatrix
              Common Stock or the Company's Acacia Research - Acacia
              Technologies Common Stock.

         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.

         For purposes of this opinion, we have examined such matters of law and
originals, or copies, certified or otherwise, identified to our satisfaction, of
such documents, corporate records and other instruments as we have deemed
necessary. In our examination, we have assumed the genuineness of all
signatures, the authenticity of all documents submitted to us as originals, the
conformity to originals of all documents submitted to us as certified,
photostatic or conformed copies, and the authenticity of the originals of all
such latter documents. We have also assumed the due execution and delivery of
all documents where due execution and delivery are prerequisites to the
effectiveness thereof. With your consent, we have relied upon certificates of
public officials and certificates of officers of the Company for the accuracy of
material, factual matters contained therein which were not independently
established.

         Based upon the foregoing and all other instruments, documents and
matters examined for the rendering of this opinion, it is our opinion that, when
issued and sold in the manner referred to in the Registration Statement, the
Securities issued and sold thereby will be legally and validly issued, fully
paid and non-assessable.

         We express no opinion as to the applicability or effect of any laws,
orders or judgments of any state or jurisdiction other than federal securities
laws and the general corporate laws of the State of Delaware and the State of
California. Furthermore, our opinion is based solely upon existing laws, rules
and regulations, and we undertake no obligation to advise you of any changes
that may be brought to our attention after the date hereof.

         We consent to the filing of this opinion letter as Exhibit 5.1 to 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, the rules and regulations of the Securities and Exchange
Commission promulgated thereunder, or Item 509 of Regulation S-K.

                                     Very truly yours,

                                     /s/ ALLEN MATKINS LECK GAMBLE & MALLORY LLP

                                     ALLEN MATKINS LECK GAMBLE & MALLORY LLP