Alloantibodies in broadly sensitized patients are difficult to characterize because they comprise mixtures with different specificities. By studying the ability of patients' sera to inhibit or enhance the binding of two anti-HLA-A2 monoclonal antibodies, BB7.2 and MA2.1, reactivity with A2 epitopes could be operationally defined. One group of patients had antibodies that inhibited the binding of both monoclonal antibodies. The inhibitory activity was removed by absorption with A2 positive cells but not with A28 or B17 positive cells. These alloantibodies apparently recognized the A2-Bw69 epitope also defined by the BB7.2 monoclonal antibody. In other patients, the inhibitory activity was removed by A2 and B17 positive cells, suggesting the involvement of the same epitope recognized by the MA2.1 monoclonal antibody. A third type of reactivity was detected in other patients. These alloantibodies enhanced the binding of BB7.2, inhibited MA2.1, and were absorbed by A2 and B17 positive cells indicating that the A2-B17 epitope was involved in these cases too. These studies underscore the potential value of monoclonal antibodies in dissecting the specificities of complex alloantibody mixtures.