The inheritance of quantitative changes in serum T4-binding globulin (TBG; reduced or elevated serum levels) and electrophoretic variants of TBG have been shown to be X-chromosome linked. However, it recently was suggested that another TBG variant, widely distributed in the Australian Aborigine population, may be inherited as an autosomal dominant trait. This communication deals with studies directed to the elucidation of the mode of inheritance of the Aboriginal variant TBG. By measuring the rate of denaturation of TBG at 56 C, we identified three distinct types of TBG in Australian Aborigines. One was a relatively heat-stable TBG (mean t1/2, 58.0 min; range, 68-53 min; group A), indistinguishable from TBG in caucasians (mean t1/2, 55.1; range, 67-43); another was a heat-labile TBG (mean t1/2, 20.8 min; range, 23.7-18.4 min; group C); and a third had intermediate values (mean t1/2, 35.7 min; range, 39.5-30.6 min; group B). Serum samples from the latter group belonged exclusively to women. Assuming that individuals from group A were homozygous for the caucasian type TBG (TBGCC), those from group C were homozygous for the Aboriginal variant of TBG (TBGAA), and individuals from group B were heterozygous (TBGCA), gene frequencies were calculated for the product of TBGC and TBGA, and the incidence of expected genotypes was compared to that observed. The results are compatible with X-chromosome, but not autosomal, inheritance, with a gene frequency of TBGC of 0.4118 and of TBGA of 0.5882. The ability to identify individuals who are heterozygous for the Aboriginal variant TBG confirmed that the structural gene of TBG in man is located on the X-chromosome.