The role of the ABO blood group system in determining the outcome of bone marrow transplantation was investigated in 53 patients with aplastic anemia and acute leukemia grafted from HLA-identical siblings. There was no correlation between ABO compatibility and marrow engraftment, graft rejection, or graft-versus-host disease. In 5 recipients with antibodies prior to transplantation to antigens of the ABH system present on the cells of their donors, plasma exchange and antibody absorption in vivo were effective in permitting engraftment of ABO-incompatible bone marrow. These findings indicate that the ABO system is not a clinically significant barrier to successful bone marrow transplantation in otherwise histocompatible individuals.