In the clinical management of patients receiving blood group ABO-incompatible organ allografts, it is of importance to determine the levels of blood group A and B antibodies before and after transplant. Currently used methods, which are mostly based on hemagglutination, are inexact and are associated with large intercenter variations. Here, we describe preliminary data from our efforts to establish a flow cytometry-based assay for the semiquantification of blood group A and B antibodies using beads carrying synthetic A or B trisaccharides. In agreement with previous investigations, blood group O individuals had greater levels of anti-A immunoglobulin G (IgG) than B individuals, whereas the levels of anti-A immunoglobulin M (IgM) were similar in sera from blood group O and B individuals.