Results of tests for human-type and simian-type blood groups on three Pan paniscus confirm their status as a separate species. Well-defined differences exist between blood group characteristics of P. paniscus and P. troglodytes. In some aspects serological red cell specificities of P. paniscus are more similar to those of man than are those of any other primate animals.

Blood grouping of nine pygmy chimpanzees revealed them to be human-type group Ai, MṜh(0), and simian-type v.D, CCef, g, H, I, K, L. Only group N^c was polymorphic. Pan paniscus red cells can be easily distinguished from those of Pan troglodytes by the serological characteristics of human-type blood groups A and M. Also, the distribution of the simian-type blood group systems V-A-B and C-E-F are strikingly different in the two species.

Twenty-one isoimmune sera produced in rhesus monkey (Macaca mulatta) containing type-specific antibodies for simian-type red cell antigens were tested for their cross-reactivity with red cells from crab-eating macaques (M. fascicularis). The majority of the antisera gave cross-reactions determining polymorphisms in the red cells of crabeating macaques, homologous to those of rhesus monkeys. These results attest to the close taxonomic relationship between the two species of macaques, and have the practical implication that isoimmune sera produced for blood typing can also be used for typing red cells from related species, as has been also observed in studies on apes.

Evidence is presented that the graded differences observed among the subgroups of A in man and nonhuman primates, and in newborn as compared with adults is due, not to differences in the nature of the determinant group nor to differences in the number of antigenic sites on the red cell envelope, but to differences in the length of the chain subjacent to the determinant group.

Blood and saliva from two mountain gorillas have been tested for human-type A-B-H, M-N and Rh-Hr blood groups as well as for homologues of the chimpanzee V-A-B and C-E-F systems. Both animals proved to be group B, and in this respect as well as in the other tests their red cells exhibited no apparent differences from the red cells of lowland gorillas previously tested by us.