Monoclonal antibody reagents (McAbs) to kangaroo red cells and the use of a simple agglutination test constitute an efficient way of answering certain taxonomic questions. Two types of antigens are recognised; a group of conservative antigens and a group of fast-evolving antigens. The former are frequently monomorphic, the latter frequently polymorphic, within a species. The conservative antigens distinguished the Macropodoidea from other marsupial families as well as the Macropodoidea from the Potoroidae. The fast-evolving were particularly useful in detecting a hybrid zone between two subspecies of Macropus robustus, M. r. robustus and M. r. erubescens in Queensland. They also showed that there is no detectable hybridisation between the two closely related species of grey kangaroo M. giganteus and M. fuliginosus in their zone of overlap. We suggest that McAbs are useful for recognising relationships between families and superfamilies, between very closely related species and subspecies, and individuals within a species. They are probably less useful for recognising intermediate relationships.