An equilibrium dialysis technique was used to measure simultaneously the proportion of free thyroxine (%FT4) and free 3,4,5'-triiodothyronine (%FT3) in sera from patients with diverse states of thyroid function and abnormal levels of plasma T4-binding proteins. In general, the correlation between %FT4 and %FT3 values was excellent in the entire group of patients studied. Studies were also conducted to ascertain whether Sephadex columns could be employed to obtain simultaneous measures of plasma binding of T4 and T3. Mixtures of diluted serum and 125I-T4 and 131I-T3 were applied to columns of Sephadex in order to separate "bound" and "free" fractions. The values for percent free T4 and T3 yielded by the Sephadex process (%FT4S and %FT3S), although far greater numerically, correlated closely with 5FT4 and %FT3 measured directly by equilibrium dialysis. When %FT4 and %FT3 were multiplied by their respective serum concentrations, the resulting free T4 and free T3 indices provided good separation of hyperthyroid and hypothyroid groups from the euthyroid group. As in the dialysis method, %FT4S closely correlated with %FT3S.