These continuing studies are designed to elucidate the immune mechanism for the specific alteration in responsiveness which is induced in certain mice by the presence of a kidney or heart transplant from a donor incompatible in regard to H-2 antigens. Transplants of kidneys and hearts using microvascular techniques are employed in the experiments and a method for the long-term intravenous infusion into mice of histocompatibility antigen preparations is being evaluated in respect of its ability to induce specific unresponsiveness. Some of the experiments make use of transplanted tumors which can grow to cause the death of allogeneic recipients in spite of the same H-2 incompatibilities concerned with surviving organ transplants. By additional donor antigen treatment, combined by BCG or cyclophophamide administration the survival of all these transplants: tumors, hearts, and kidneys can be abruptly brought to an end by an induced specific immune reaction, the nature of which will be studied by in vivo and extensive in vitro tests.