Monoclonal antibodies, because of their specificity and unlimited availability, have become one of the most powerful experimental tools available to the biological sciences. It is possible to make monoclonal antibodies that bind to determinants that are monomorphic in one or more species or to determinants that are polymorphic within a species. Few monoclonal antibodies have been made using immunogens derived from nonhuman primates. However, some monoclonal antibodies that recognize monotypic markers in humans can be used to detect polymorphic markers in nonhuman primates. Thus, the rapid development of monoclonal antibodies specific for human proteins significantly increases the potential number of immunogenetic markers useful for studying phylogenetic relationships and for identifying genetic polymorphisms among nonhuman primates.