Histocompatibility antigens (HCA) have been solubilized by the use of Triton X-100 andn-butanol from cell membrane fragments of spleens and livers of BALB/c and CBA mice. These soluble preparations could inhibit or neutralize allogeneic antibody (CBA anti-BALB or BALB anti-CBA) in two biological tests, cytotoxicity and the suppression of antibody formation by appropriately stimulated spleen cells of the same strain. The neutralizing activities of the spleen-derived preparations, per milligram, were about 10 times those derived from liver, and preparations extracted by butanol or by papain were of substantially higher specific activity than those extracted by Triton X-100. All preparations contained active molecular species excluded by Sephadex G-200 and others, nonexcluded, in various proportions. Both Sephadex fractions could be adsorbed to sheep red blood cells (SRBC) but only the adsorption of the G-200-excluded material rendered the SRBC agglutinable by CBA anti-BALB antibodies; the non-excluded fraction could compete with the excluded fraction to prevent agglutination of the coated SRBC by such antibody. The soluble antigen could inhibit the allogeneic antibody in two serological tests, the adsorption hemagglutination (AH) reaction referred to, and the standard hemagglutination test of BALB RBC by CBA anti-BALB antibody.