BALB/e histocompatibility antigens (HCA), solubilized from cell fragments of spleens and livers by Triton X-100, butanol, and papain, were tested for immunogenicity as transplantation antigens by injection into normal CBA mice and later observation of the time of rejection of BALB skin grafts in these mice. Each type of preparation, injected 3 times in saline or in oil adjuvant, caused accelerated rejection of such grafts. Preparations solubilized by butanol or papain showed higher specific activity than the Triton-solubilized, and preparations from spleen showed higher activity than those from liver. These differences were similar to the differences in specific activity shown by the respective preparations in the inhibition of 2 effects of CBA anti-BALB antibody, cytotoxicity, and the suppression of hemolytic antibody plaque production by BALB spleen cells. CBA antigen solubilized by butanol from liver cell fragments similarly caused accelerated rejection of CBA skin grafts in BALB mice following injection on the same schedule.