HL A and mixed lymphocyte culture identical sibling bone marrow can be engrafted following cyclophosphamide, total body irradiation and B.A.C.T. chemotherapy (4 day cyclophosphamide regimen in combination with 2 agents considered to be effective for treatment of acute myelocytic leukemia, cytosine arabinoside and 6 thioguanine, together with a fourth drug, bis chlorethyl nitrosurea). HL A identical bone marrow can be engrafted across the ABO erythrocyte antigen barrier. B.A.C.T. appears to provide better tumor ablation prior to marrow transplantation and sufficient immunosuppression to produce complete donor chimerism, as compared to cyclophosphamide alone. The 6 day version of the protocol may produce undesirable side effects attributable to the additional cyclophosphamide employed. Current studies in progress will examine the efficacy of the 4 day B.A.C.T. regimen in conjunction with posttransplant treatment with methotrexate for the prevention of graft vs. host disease. In situations where clinically severe graft vs. host disease occurs, patients are treated with antilymphocyte serum.