Five patients with aplastic anemia were shown to be sensitized in vivo to the transplantation antigens of HL-A identical and MLC nonreactive sibling bone marrow transplantation donors. The sensitization in four cases were detected by antibody-dependent cell-mediated lysis (AICML) techniques, and in one case by antibody-dependent cell-mediated lysis (ADCML). In two cases the inherited nature of the sensitizing antigen was demonstrated, and in one case the inheritance of the antigen was shown by a family study to be independent of the HL-A and MLC loci. Maximal AICML was demonstrated in two cases at the time of bone marrow graft rejection. Successful transplants after immunosuppression with rabbit anti-thymocyte serum, procarbazine, and cyclophosphamide were performed in two patients, in whom sensitization was detected before transplantation.