Twenty-five patients with major ABO blood group incompatibility between donor and recipient underwent allogeneic bone marrow transplantation using erythrocyte depletion of the bone marrow infusate prior to administration. Over 95% of the original erythrocyte content of the marrows was removed, while retaining 75% of the mononuclear cell content and 57% of the granulocyte-monocyte colony-forming units. Recipients, well hydrated and premedicated with corticosteroids, diphenhydramine, and mannitol, tolerated infusions well. The frequency of engraftment, rate of recovery of peripheral blood leukocytes, granulocytes, and platelets, and the incidence of graft-versus-host disease was similar to that observed following ABO blood group compatible bone marrow transplantation. Erythroid development following ABO blood group incompatible transplantation was significantly impaired until hemagglutinins fell to 1:4 or lower, at which time recovery of erythrocytes was detected in the peripheral blood. The erythrocyte hypoplasia associated with incompatible hemagglutinins was temporary. Erythrocyte purging is a safe and effective technique to perform bone marrow transplantation across major ABO blood group incompatibilities.