Serum samples from Micronesian populations on the Pingelap, Mokil, Ponape and Kusaie islands were tested for the immunoglobulin G (IgG) allotypes, Gm (1, 2, 3, 5, 6, 13, 14, 21), and for Inv (1). All four populations have the Gm phenogroups, Gm1,21, Gm1,2,21, Gm1,3,5,13,14, and Gm1,5,6. The Ponapeans have Gm1,5,13,14 also. Pedigree analysis shows that the Gm1,5,6 phenogroup in the Pingelap and Mokil populations is derived from the single offspring of a member of a crew of a whaling ship and that the Gm1,2,21 phenogroup was introduced by three non-native individuals. The Gm allotypes indicate that the Ponapean and Kusiean populations also have phenogroups from other races, and historical data show that there has been adequate opportunity for this to have occurred. Only the phenogroups Gm1,21 and Gm1,5,13,14 appear to be endemic to eastern Micronesia.