We report a case of accelerated acute rejection of a renal allograft from a presumed ABO histo-blood group A2donor in an O recipient, in which all of the published criteria for compatibility had been met. Flow cytometric analysis of the A and H antigen expression on the kidney donor's erythrocytes suggested that this donor did not have an A2phenotype, but rather another subgroup of A. Some of the reported cases of accelerated acute rejection of A2renal allografts in O recipients may have resulted from mis-application of the results of standard lectin agglutination to the transplant setting. The current case suggests that a more sophisticated method of ABO phenotyping, such as erythrocyte flow cytometric analysis, may be necessary in the transplant setting.