Blood samples from 45 Usiai and 138 Manus people on the Island of Manus, New Guinea, have been tested for the ABO, MN, Ss, Rh, P, Duffy, Kell and Gerbich blood-group systems, the haptoglobin, transferrin, protease-inhibitor, caerulo plasmin and albumin serum-protein systems, and the acid-phosphatase, 6-phosphogluconate dehydrogenase, phosphoglucomutase (locus 1 and 2), peptidase A and B, glucose-6-phosphate dehydrogenase, lactate dehydrogenase, malate dehydrogenase, adenylate kinase, phosphohexose isomerase, adenosine deaminase, phosphoglyce-rate kinase and oxidase red-cell enzyme systems, as well as abnormal haemoglobins. The phenotype and gene frequency distributions, in general, are within the range for New Guinea and Micronesian populations, but some results are of special interest. In the Duffy blood-group system, two Fy (a–b+) persons were found, resulting in a relatively high Fyb gene frequency. In addition, the frequency of Ge(a–) persons in the Gerbich blood-group system is lower than that found recently for many other Melanesian-speaking populations in New Guinea. In the phosphoglucomutase (PGM) system, there were no examples of the PGM31 and PGM71 alleles on Manus, though these alleles are frequently present in other SW Pacific populations. Similarly in the phosphoglycerate kinase system, no examples of the PGK2 or PGK4 alleles were found on Manus, though these are present in Micronesian and among some populations in New Guinea. Finally, no D1 transferrin variants were found on Manus, but a new transferrin allele was found, with a frequency of 0.0164. This allele has been given the trivial name DManus. The results are discussed in relation to the linguistic differentiation between Usiai and Manus populations and to the intermarriage between them.