Human plasma samples, radiolabeled with [125I]thyroxin, from the Asian, Pacific, and Australian area have been subjected to isoelectric focusing to reveal genetic variation in thyroxin-binding globulin (TBG). A genetically determined electrophoretic slow variant, TBG S, indistinguishable from the variant found in black Africans, has been observed with a frequency of 1%-10% in all Melanesian and Polynesian populations studied. The TBG S variant is present also with low frequency in Micronesians and in some Indonesian populations. However, East Asians, Indians, and Australian Aborigines were found to be monomorphic.