A collection of sera from 81 species of Australasian marsupials and 65 antisera to them, prepared in rabbits, chickens, or various species of marsupials, was examined to devise a scheme to identify the blood of marsupials imbibed by blood-sucking insects. The precipitin test was of limited value but established that the blood was marsupial. With the haemagglutination inhibition test, however, it was possible to identify the family, subfamily, genus, and species. Thus a procedure could be devised to identify the donors' blood based principally on serological methods supplemented by data on the geographical distribution of the various marsupial species.