Ten monoclonal antibodies (MAbs) have been produced from mouse-mouse hybridomas raised to dispase-treated human group AB red blood cells (RBCs). One of these antibodies, T-1435A, reacted with the RBCs of the human group A, B, and AB, but not to the RBCs of group O. The reactivity of T-1435A was specific to a single anti-A, B but not to a mixture of an anti-A and an anti-B. Also, T-1435A reacted strongly with protease- and neuraminidase-treated RBCs, and the susceptibility of its corresponding antigen was markedly reduced by alpha-galactosidase treatment. T-1435A agglutinated group A RBCs from chimpanzees, and its reactivity to RBCs was recognized in tamarins and in owl monkeys. However, this antibody demonstrated a stronger affinity for human RBCs. Further, using A- or B-transferase, human and chimpanzee RBCs that transferred from group O to A or to B were agglutinated by T-1435A. Finally, the corresponding antigen appears to have a structural determinant that fits the T-1435A combining site. Therefore, this monoclonal antibody is useful for determining the ABO blood-grouping when used as a conventional reagent that segregates group O RBCs from the RBCs of the other groups.