Soluble H-2 transplantation antigens were prepared from H-2d L1210 leukemia cells by sonication and an (NH4)2SO4 precipitation of this soluble material was used to immunize rabbits. The antisera produced reacted broadly in a cytotoxic test with the spleen and tumor cells of many different inbred mouse strains, but, after absorption, reacted specifically with several H-2d alloantigens. “K region” antigens were more immunogenic than “D region” antigens, especially the “private” specificity H-2.31. The rabbit anti-mouse sera contained no detectable non-H-2 antibodies nor antibodies reactive with tumor specific antigens and was unable to form precipitin lines in agar gel. The immunosuppressive activity of this serum, tested by skin grafting, was negligible.