Human tissue isoantigens were detected on chimpanzee leukocytes and on a continuous heteroploid cell line of chimpanzee origin. The antisera used to detect these isoantigens were taken from multiparous women, multiparous chimpanzees, and chimpanzees immunized with human leukocytes and platelets. These sera were known to contain antibodies to human leukocyte isoantigens. The methods used to demonstrate the isoantigens were leukocyte agglutination and inhibition of leukocyte agglutination, as well as mixed agglutination and inhibition of this reaction.