A new method for the assay of free thyroid hormones in human serum is described. This method is based on a chromatographic adsorption process of thyroid hormones onto a Sephadex LH-20 resin column. Protein fractions are eliminated by washing columns, adsorbed hormones are eluted with methanol and determined by radioimmunoassay. It was demonstrated that under the experimental conditions adopted the presence of the resin R does not significantly change the free hormone level in the serum, and the amount of hormone adsorbed onto the resin HR is exclusively in function of the free hormone concentration [H], according to a linear relationship: HR = phi [H], where phi is the resin adsorption constant K ads multiplied by the number of resin binding sites nR. The phi value, experimentally determined, was 32 ml for T3 and 58 ml for T4, when 150 mg resin were used. The method sensitivity was 0.3 pg/ml for FT3 and 0.6 pg/ml for FT4. The within-assay reproducibility was about 5% (CV) and the between-assay reproducibility was about 6% (CV), both for FT3 and FT4. FT3 and FT4 levels, in 96 normal subjects, were 3.9 +/- 0.7 pg/ml (mean +/- SD) and 11.1 +/- 1.9 pg/ml (mean +/- SD) respectively.