[Letterhead of O'Melveny & Myers LLP]



January 25, 2000


Acacia Research Corporation
55 South Lake Avenue
Pasadena, California 91101

             Re:  Registration Statement on Form S-3
                  ----------------------------------

Ladies and Gentlemen:

     At your request, we have examined the Registration Statement on Form S-3
(the "Registration Statement") to be filed by you with the Securities and
Exchange Commission on January 25, 2000 in connection with the registration
under the Securities Act of 1933, as amended, of 2,345,116 shares of your
Common Stock, $0.001 par value (the "Shares").

     We are familiar with the proceedings heretofore taken, and with the
additional proceedings proposed to be taken, by you in connection with the
authorization and proposed issuance and sale of the Shares.

     It is our opinion that, subject to said proceedings being duly taken and
completed by you prior to the issuance and sale of the Shares, upon the
issuance and sale thereof in the manner referred to in the Registration
Statement, the Shares will be legally and validly issued, fully paid and
nonassessable.

     We consent to the use of this opinion as an exhibit to the Registration
Statement.

                                                Respectfully submitted,
                                                /s/ O'Melveny & Myers