Six cases of immune hemolytic anemia attributed to donor-derived red cell antibodies after allogeneic bone marrow transplantation (BMT) are reported. In 2/6 cases, severe intravascular hemolysis was seen, 6/6 required increased red cell transfusion, and 1/6 was treated by plasma exchange. All recipients were receiving cyclosporine to prevent graft-v- host disease. Investigations showed that in each case, the donor lacked ABO or Rho(D) red cell antigens present in the recipient. The direct antiglobulin test was positive in 6/6. Relevant serum antibody (anti-A, four cases; anti-B, one case; anti-D, one case) was first detected one to three weeks after BMT. Eluates made from recipient red cells showed the same specificity as serum antibody. Maximum hemolysis occurred nine to 16 days after BMT, suggesting that active production of antibody by “passenger” donor lymphocytes was the likely mechanism of hemolysis, rather than passive transfer of antibody in the marrow infusion. Retrospective analysis of 21 consecutive cyclosporine-treated BMT patients receiving marrow lacking ABO or D antigens present in the recipient showed that (1) 15/18 patients tested had red cell antibody production against recipient red cell antigens; (2) despite the frequent presence of antibody specific for recipient red cell antigens, only 3/21 patients developed clinically significant hemolysis; (3) clinical hemolysis could not be predicted by donor or recipient red cell antibody titers. We conclude that although red cell antibody against recipient antigens is frequently produced after minor ABO and D mismatched BMT in cyclosporine-treated recipients, only 10% to 15% of cases develop clinically significant immune hemolysis. The data presented show that the most likely source of antibody is “passenger” donor lymphoid cells.