A euthyroid, 14-year-old boy of Jewish Syrian origin was found to have a severe deficiency of thyroxine-binding-globulin (T.B.G.). The same defect was also found in his brother and maternal uncle, and to a lesser degree, in his sister, mother, maternal aunt, grandmother and the uncle's daughter. These findings are consistent with the wellknown X-linked mode of inheritance. Investigations of the family for other X-linked markers revealed severe deficiency of G-6-PD in the maternal uncle and partial G-6-PD deficiency in the proband's mother, aunt, grandmother and the uncle’s daughter. To our knowledge, this is the first family in which these two traits were found to segregate. The finding of two X-linked defects in the same family is of value in the mapping of the gene on the X-chromosome. From the investigation of this family it is concluded that if the locus of T.B.G. is indeed on the X chromosome, then it is not closely linked to that of G-6-PD.