Ninety multitransfused patients with severe aplastic anemia were conditioned with cyclophosphamide and given marrow transplants from HLA-identical siblings. Thirty patients had complement-dependent lymphocytotoxins against a panel of cells from unrelated individuals immediately before transplantation while 60 did not. Lymphocytotoxins were more frequent among patients who had received more blood product transfusions, and these patients were more likely to be refractory to random donor platelet support. Twenty-seven patients rejected the marrow transplant while 63 had sustained engraftment. The presence of pretransplant lymphocytotoxins did not correlate with graft rejection and hence does not appear to be useful as a test for identification of patients at high risk of rejection.