Serum thyroxine binding globulin (TBG) was measured with a radioimmunoassay (RIA) kit (GammaDab TBG). The TBG concentration in 23 adult normals was $23.7{pm}1.9{mu}g/ml(mean{pm}SD)$. The serum TBG levels of $21.6{pm}3.5{mu}g/ml$) in hyperthyroidism, $24.7{pm}4.9{mu}g/ml$ in subacute thyroiditis, $20.7{pm}7.0{mu}g/ml$ in liver cirrhosis and $22.6{pm}3.7{mu}g/ml$ in sick patient were not significantly different from normals. The levels of $31.8{pm}5.9{mu}g/ml$ in hypothyroidism, $36.2{pm}5.1{mu}g/ml$ in pregnancy (p<0.01, p<0.001) and $29.3{pm}6.1{mu}g/ml$ in molar pregnancy (p<0.01) were significanty higher that in normals. In various cases without thyroid dieases (euthroid group), the TBG concentration correlated with the value for Amerlex $T_3$ (r=0.816) though there was curvilinear relationship. This relationship was altered in hyperthyroidism, subacute thyroiditis and molar pregnancy in which sera were overloaded with thyroxine $(T_4)$ so that concentration of unoccupied binding sites on TBG (free TBG concentration) were more decreased than expected from normal TBG concentrations. Hypothyroidism was also separated from the curvilinear relationship in euthyroid group indicating that free TBG concentrations were more increased relative to slightly increased TBG concentrations. Measurement of the TBG concentration was considered useful in the diagnosis of TBG defiency, in differentiating molar pregnancy from hyperthyroidism and for correct understanding the hormone binding in liver dieases and other nonthyroidal illness.