The phenomenon of accommodation in recipients of blood group ABO incompatible kidney transplantation (iKTx), in which grafts survive despite the presence of blood group A or B antigen in the graft and the presence of corresponding antibodies in the recipient's blood, is not uncommon. alpha1,3-N-Acetylgalactosaminyltransferase and alpha1,3galactosyltransferase associated with the synthesis of blood group A and B antigen (A and B enzymes), respectively, were measured by a highly specific enzyme-linked immunosorbent assay in the sera and transplanted tissues of patients who underwent an ABO iKTx. Allogeneic A and B enzymes were present in the sera and tissues as well as A and B antigens in the tissues for a long period, which hitherto have never been seen in recipients prior to an iKTx. However, activities of these enzymes in the sera after an iKTx decreased in patients who experienced a serious acute antibody-mediated rejection and disappeared in patients who had an unrepairable rejection, leading to graft loss without establishment of accommodation. Our observations on the presence of allogeneic A and B enzymes in the recipients' sera should have implications in decision making for a successful iKTx.