Soluble antigen prepared by Triton and butanol from cell membrane fragments of rabbit lymph node and spleen has been shown to be associated with the suppression or rejection of allogeneic lymph node cells by its abilities (1) to inhibit or absorb the suppressive antibody in rabbit antileucocyte serum, (2) to induce actively in normal rabbits the rejection of transferred lymph node cells, and (3) to stimulate production of antibodies suppressive for allogeneic lymph node cells. This can be demonstrated in cell transfer or, in vitro, in the hemolytic antibody plaque test. Analysis by chromatography and sucrose density gradient ultracentrifugation shows that the rejection-associated antigen can be found within approximately half the total protein of the butanol extract, and suggests nonidentity of the antigen detectable by inhibition of suppressive antibody and that detectable serologically. Further evidence of nonidentity of the antigens and antibodies detected is given by substantial disproportionality of titers determined in suppressive and serological tests in primary and secondary antisera to whole allogeneic cells, respectively, and in secondary and tertiary antisera to the butanol-solubilized antigen.