SUMMARY The immunogenic properties of papain-solubilized alloantigen preparations from C57EL/6.I and BALB/c+I spleen cells were studied. Doses of 2.S-50 μg of protein, administered with Freund's adjuvant, accelerated skin graft rejection across H-2 barriers. In addition, BALB/cJ preparation was shown to possess non-H-2 activity. The i.v. injection of single and multiple doses of the C57BL/6J preparation did not prolong graft survival. Some groups even showed accelerated rejection.