The survival of chromium-labeled group A erythrocytes was measured in a patient who had previously received two bone marrow transplants for reconstitution of lymphopenic hypogammaglobulinemia. The patient was of blood group A before transplantation; the donor of blood group O. The patient’s erythrocytes are now virtually 100% group O. Anti-B titers are present; anti-A antibodies are not demonstrable. The cells producing these isohemagglutinins are of donor origin; the donor has anti-A titers of 1:64 in saline and 1:256 by antihuman globulin test as well as anti-B titers. No evidence of immunological destruction of group A erythrocytes was found in this patient suggesting that the immune system of the donor may have become tolerant of the group A substance of the recipient.