Enzyme--monoclonal antibody complexes formed between six different monoclonal antibodies and the six phenotypes of human placental alkaline phosphatase [orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] that represent the homozygous and heterozygous combinations of the three common alleles have been examined by electrophoresis in starch, acrylamide, and agarose gels. Since the complexes formed retain full enzyme activity, they could be detected after gel electrophoresis by an enzyme stain. Distinctive electrophoretic patterns were obtained with each monoclonal antibody. Differential binding of certain of the antibodies with the products of different alleles produces clear discrimination of various homozygous and heterozygous phenotypes. This discrimination parallels the results previously obtained by using a quantitative binding radioimmunoassay. The results show that this general method should prove useful in screening hybridoma fluids for the presence of monoclonal antibodies to specific enzymes; in the detection of allelic variation, even where this is not expressed by electrophoretic differences among the uncomplexed enzymes; and in discriminating between homozygotes and heterozygotes. It could also prove to be a useful tool in the elucidation of the molecular structures of enzyme--monoclonal antibody complexes.