A case of acute myelocytic leukemia has been reported in which the patient's surviving original B lymphocytes after pretransplant-conditioning chemotherapy probably reproduced hemagglutinins that reacted with red blood cells (RBCs) derived from engrafted donor marrow for a prolonged period of time. Although the direct antiglobulin test was negative and hemagglutinins were not detectable in the patient's sera but only in the eluate, the antibodies reappeared in the sera. Therefore, it is important to confirm that the eluate does not contain antibodies that would react with donor-derived RBCs when the type of red cell used for transfusion is switched from the patient's type to the donor's type in a major ABO-mismatched bone marrow transplantation (BMT). Testing of ABO subgroups using lectins is also recommended to avoid a delayed hemolytic transfusion reaction following BMT.