EXHIBIT 5.1

                  OPINION AND CONSENT OF GREENBERG TRAURIG, LLP

                                                              September 15, 2005

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 acted as counsel to Acacia Research Corporation, a Delaware corporation
(the "Company"), in connection with the filing of a Registration Statement on
Form S-3 (the "Registration Statement") pursuant to Rule 462(b) of Regulation C
promulgated under the Securities Act of 1933, as amended (the "Securities Act"),
relating to the offering and sale by the Company from time to time of shares of
its Combimatrix Common Stock, par value $0.001 per share ("Common Stock"), of
the Company (the "Shares"). The Shares are being registered for offering and
sale from time to time pursuant to Rule 415 of the General Rules and Regulations
promulgated under the Securities Act (the "Securities Act Rules"). The
Registration Statement incorporates by reference the Registration Statement on
Form S-3 (No. 333-112885), which was filed on February 17, 2004 and which has
been declared effective by the Securities and Exchange Commission, including the
prospectus which forms a part of such registration statement (the "Prospectus"),
as supplemented from time to time by one or more prospectus supplements (each, a
"Prospectus Supplement").

In connection with this opinion, we have examined and relied upon the originals,
or copies certified to our satisfaction, of such records, documents,
certificates, opinions, memoranda and other instruments as in our judgment are
necessary or appropriate to enable us to render the opinion expressed below. As
to certain factual matters, we have relied upon certificates of the officers of
the Company and have not sought to independently verify such matters.

In rendering this opinion, we have assumed the genuineness and authenticity of
all signatures on original documents; the authenticity of all documents
submitted to us as originals; the conformity to originals of all documents
submitted to us as copies; the accuracy, completeness and authenticity of
certificates of public officials; and the due authorization, execution and
delivery of all documents where authorization, execution and delivery are
prerequisites to the effectiveness of such documents. We have also assumed that,
at the time of issuance and sale, a sufficient number of shares of Common Stock
is authorized and reserved or available for issuance and that the consideration
for the issuance and sale of the Shares is not less than the aggregate par value
of the Shares, and that, prior to any offering and sale of the Shares, the
Company's board of directors (the "Board"), including any appropriate committee
appointed thereby, will duly authorize the terms of and the prices at which the
Shares are to be issued and sold.

Our opinion herein is expressed solely with respect to the federal laws of the
United States and the Delaware General Corporation Law. We express no opinion as
to whether the laws of any jurisdiction are applicable to the subject matter
hereof. We are not rendering any opinion as to compliance with any federal or
state antifraud law, rule or regulation relating to securities, or to the sale
or issuance thereof.

On the basis of the foregoing and in reliance thereon, and subject to the
qualifications herein stated, we are of the opinion that, with respect to any
offering of the Shares, if (i) the Registration Statement has become effective;
(ii) the Prospectus and any Prospectus Supplement(s) required by applicable laws
have been delivered as required by such laws; (iii) the Shares are to be sold
pursuant to a purchase, underwriting or similar agreement, such purchase,
underwriting or similar agreement has been duly authorized, executed and
delivered by the Company and the other parties thereto and has become a valid
and binding agreement of the Company; (iv) the Board, including any appropriate
committee appointed thereby, and appropriate officers of the Company have taken
all necessary corporate action to approve the issuance and terms of the Shares
and related matters; and (v) the terms of the Shares and of their issuance and
sale have been duly established in conformity with the operative certificate of
incorporation and bylaws of the Company and the Delaware General Corporation Law
so as not to violate any applicable law, the operative certificate of
incorporation or bylaws of the Company or result in a default under or breach of
any agreement or instrument binding upon the Company and so as to comply with
any requirement or restriction imposed by any court or governmental body having
jurisdiction over the Company, then the Shares, when issued and sold in
accordance with a duly authorized, executed and delivered purchase, underwriting
or similar agreement, will be duly authorized, validly issued, fully paid and
nonassessable.

We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to our firm under the caption "Legal
Matters" in the Prospectus included in the Registration Statement.

Very truly yours,


/s/ Greenberg Traurig, LLP
Greenberg Traurig, LLP