Etoposide and cytarabine have been shown to exert high antileukemic activity and are currently under study as preparatory agents before allogeneic bone marrow transplantation. However, data concerning their engraftment-promoting potency are scarce. Therefore, we tested these agents in LEW rats receiving a myeloablative dose of busulfan followed by transfer of F1 (CAP X LEW) marrow, which is unable to induce a graft-v-host reaction (GVHR). Since busulfan by itself has only minor immunosuppressive potency, graft rejection ensues unless etoposide, cytarabine, or cyclophosphamide provide additional immunosuppression to facilitate durable engraftment. Before allogeneic bone marrow transplantation in humans, 120 mg/kg of cyclophosphamide, 60 mg/kg of etoposide, or 900 mg/kg of cytarabine are the standard doses given in conjunction with total body irradiation. Seventy-five percent of these doses administered in addition to busulfan resulted in rejection rates of 75% for cytarabine and 58% for etoposide, respectively, whereas no rejections were observed with cyclophosphamide. These data indicate that etoposide and cytarabine are inferior to cyclophosphamide in their rejection-preventing potential. Using either of these agents as substitutes for cyclophosphamide before allogeneic bone marrow transplantation may increase the risk of graft rejection in HLA-mismatched bone marrow transplantation and, in case of HLA identity, if T-depleted marrow is administered.