We have evaluated a new immunoradiometric assay for free thyroxine (T4) utilizing thyroxine-coated particles and radioiodinated monoclonal anti-T4 antibodies. The assay results were reproducible with coefficients of variation no more than 3.1% for intraassay and no more than 6.7% for interassay. Serum free T4 values for 48 healthy subjects distributed at 1.13 +/- 0.19 ng/dl (mean +/- SD). In various thyroid diseases, the results with the new kit compared well with two other measurements (T4-analog RIA and equilibrium dialysis RIA). Altered concentration of thyroxine binding globulin did not significantly affect this assay, while anti-thyroxine autoantibodies caused factitiously elevated free T4 values in some cases. In pregnancy, free T4 concentration tended to decline mildly in late stages. About 20% of hypoalbuminemic samples from patients with nonthyroidal illnesses had free T4 concentration more than 2.00 ng/dl, probably due to renal failure or to the use of diuretics, heparin, or other drugs which may increase serum free T4 fraction in vivo. In conclusion, this new free T4 IRMA kit is convenient and reproducible, and suitable for routine measurement of clinical material.