Bone marrow transplantation results in the infusion of 150 to 360 ml of erythrocytes. While this poses no problem to a recipient of marrow from an ABO-compatible donor, it clearly represents a serious risk of intravascular hemolysis for the recipient of an ABO-incompatible marrow. Thus, there is a need for removing incompatible erythrocytes from the marrow preparation. We removed erythrocytes from the marrow preparations by automated centrifugation. The erythrocyte-depleted marrow (EDM) contained a mean of 5 ml erythrocytes, representing an average reduction of 98%. The mean mononuclear cell recovery was 8 8 % , resulting in a mean infusion of 0.6 X lo8 cells/Kg of recipient's body weight in a final average volume of 155 ml. EDM was infused into 22 ABOincompatible marrow recipients (21 patients with hematologic malignancies and one patient with aplastic anemia) without clinical evidence of hemolysis. The isohemagglutinin titers of recipients ranged from 4 to 4096 and were not lowered prior to infusion. Engraftment (i.e.. recovery of peripheral leukocyte and platelet counts) and incidence of graft versus host disease were similar to those observed in recipients of ABO-compatible marrow transplantation. Erythrocyte engraftment was significantly delayed in only one patient who had a high isohemagglutinin titer. The post-transplantation red cell requirement was increased in EDM recipients: 9 units compared to 6 units in ABOcompatible bone marrow transplanted patients with neither hemolysis nor interference with successful engraftment .