Mutations of the thyroid hormone receptor (TR) beta 1 gene have recently been detected in several unrelated families with generalized resistance to thyroid hormone (GRTH). We now report a novel point mutation in the TR beta 1 gene in a case of a Korean-Japanese kindred. The intracellular localization and the amount of TR proteins were considered to be normal by the immunocytochemical study of cultured skin fibroblasts from the patients using anti-T3 receptor antibody. The cDNA of the T3-binding domain of the TR beta 1 gene, synthesized from the total RNA of the patients' fibroblasts, was amplified by the polymerase chain reaction, and was sequenced. A point mutation, A to G, in one allele at 1612 resulting in an amino acid substitution from lysine 438 to glutamic acid was detected. The same mutation was identified in one allele in each of the affected members. In vitro translation products of the mutant TR beta 1 gene showed decreased T3-binding activity. These data suggest that a TR mutation is predominantly responsible for GRTH, irrespective of ethnic background.