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.

98 Mexican Indians were tested for the blood properties A-B-O, A1-A2, M-N, P, Rh'-Rh''-Rh0-rh, and Hr. Of the 98 Indians, 90.8 per cent belonged to group 0, 6.1 per cent belonged to A1, and 3.1 per cent to group B. There were 61.2 per cent of type M, 3.1 per cent of type N, and 35.7 per cent of type MN. Of the 95 Mexican Indians tested with anti-P serum, 21.1 per cent were found to lack the P agglutinogen. In tests for the Rh blood types, 48.0 per cent of the Indians were found to belong to type Rh1, 9.2 per cent to type Rh2, 41.8 per cent to type Rh1Rh2, and 1 per cent to type Rh0. There were no bloods giving intermediate reactions. Of the 95 Indians tested for the Hr factor 44.2 per cent were found to lack this property. The reactions for the Rh blood types and Hr factor were correlated with each other and the results supported the conclusion of Race et al. that in addition to the six standard allelic genes and the so called intermediate genes, there is one or possibly two genes having the property of determining agglutinogens which react with anti-Rh' and anti-Rh'' sera, but not with anti-Hr serum. This gene (or genes) appears to be relatively common among Mexican Indians (approximately 3.3 per cent) in contrast to its rareness in white individuals.

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.