We describe one-step labeled-antibody assays for measuring free T4 and free T3 concentrations in serum, based on a novel principle (Amerlex-MAB). Free T4 or free T3 in the sample competes with a molar excess of a cross-reactant (T3 or T2, respectively), chemically coupled to magnetizable polymer particles, for binding to 125I-labeled monoclonal anti-T4 or anti-T3 antibody, respectively. 125I radioactivity bound to the solid phase is inversely proportional to the serum free T4 or free T3 concentration. This one-step assay apparently proceeds to equilibrium after 30 min at 37 degrees C. Within- and between-assay precision (CV) was < 5.7% for free T4 or 6.2% for free T3. The reference range was between 0.98 and 1.77 ng/100 ml for free T4 and between 2.8 and 4.6 pg/ml for free T3. The measurement of free T4 and free T3 concentrations could clearly discriminate hyperthyroid and hypothyroid patients from euthyroid subjects. These values correlated closely to those obtained by an analog radioimmunoassay (Amerlex-M). This method is free from interference by major T4-binding proteins in serum, showing improved performance, compared to the analog radioimmunoassay, with sera from NTI patients with low serum albumin concentrations or anti-thyroid hormone antibodies. We expect these assays to be clinically useful for the evaluation of thyroid functions.