Donor-recipient AB0 incompatibility occurs in up to30% of all hematopoietic stem cell transplantations(HSCT) [1]. Immunohematologic complications canarise from major AB0 incompatibility, where donor redblood cells (RBC) are attacked by recipient isohemag-glutinins (anti-A and/or anti-B). Complications can alsoresult from minor incompatibility in which donor plasmacontains antibodies directed against recipient RBCantigens. Plasma can be removed from the graft in orderto prevent rapid hemolysis from passive transfer ofisohemagglutinins in the case of minor AB0 incompat-ibility. Delayed immune mediated hemolysis may none-theless occur, usually 1–3 weeks after transplantation,concomitant with myeloid engraftment [1–3]. Theprovenance of these transient AB0 antibodies has beenattributed to donor-derived ‘memory’ B lymphocytestransfused with the graft (‘passenger lymphocytes’).In 1999, a 7-year-old boy (bloodgroup A RhD⁄)inthird untreated relapse of pre-B acute lymphoblasticleukemia received a bone marrow stem cell graft from aone antigen-mismatched (HLA-C allele) unrelated maledonor (bloodgroup 0 RhD⁄). The donor had no history ofprevious blood transfusions, and screening for irregularRBC antibodies was negative in both donor and recipient.After conditioning with etoposide (30 mg/kg), cyclopho-sphamide (120 mg/kg) and fractionated total bodyirradiation (2 Gy b.i.d. 6‹12 Gy) the patient received400 ml of plasma depleted bone marrow containing atotal of 7.94 10