The ratio of fetal hemoglobin to total hemoglobin (%Hb F), the ratio of G gamma to total gamma globin (%G gamma), and the polymorphism of the XmnI site at -158 base pairs from the cap site of the G gamma-globin gene were examined in normal unrelated Japanese (n = 113) and Korean (n = 44) adults. The frequency of the presence of the XmnI site was 0.15 in the Japanese and 0.16 in the Korean population. There were statistically significant differences in the %G gamma values of the Japanese between those +/+ and those +/- or -/- at the XmnI site (p less than 0.01). The Korean %G gamma values showed a statistically significant difference (p less than 0.01) between those +/- and those -/-. The presence of the XmnI site was significantly associated with the elevation of G gamma-globin chain synthesis, but this relationship was not necessarily absolute. The absence of the XmnI site in an adult with a gamma-globin gene triplication (G gamma AG gamma A gamma/G gamma A gamma) more or less reduced the level of G gamma-globin chain synthesis, but the presence of the XmnI site in an adult with a gamma-globin gene deletion (GA gamma/G gamma A gamma) had no effects on the proportion of the two gamma-globin chains.