The identification of a number of leukocyte antigens of chimpanzees is described. Suggestive evidence was obtained that some of the isoantisera recognize specificities of a hypothetical genetic system of chimpanzee antigens which, in analogy with the humanHL-Asystem, is tentatively calledChL-A.We demonstrated the similarity of severalChL-Aspecificities with human antigens of theHL-Asystem by typing chimpanzee cells with monospecific human sera and human cells with chimpanzee isoantisera. Several chimpanzee sera contained antibodies of distinctHL-Aspecificity when used for human tissue typing (HL-A1, 7, and 11); antigens 4a and 4b appear to be distributed in an alternate fashion, also, on cells of unrelated chimpanzees.