We have designed a re-usable dialysis cell and a complex dialysis buffer, with which undiluted serum samples can be dialyzed with minimal changes in their serum matrix. Dialysate thyroxin (free T4) is then measured by a sensitive RIA for T4. The range of reportability was 2-128 ng/L, the normal range was 8-27 ng/L, and the interassay CV was 7%. Free T4 concentrations in various disorders were as follows: hyperthyroidism, 32-478 ng/L; in both excess thyroxin-binding globulin (TBG) and familial dysalbuminemic hyperthyroxinemia, 9-27 ng/L; primary hypothyroidism, less than 2-7 ng/L; central hypothyroidism, 4-6 ng/L; severe TBG deficiency, 9-25 ng/L; hypothyroxinemias of nonthyroidal illness, 8-35 ng/L. With this free-T4 assay, which is adaptable to clinical laboratory use, one can differentiate hyperthyroidism from the major euthyroid hyperthyroxinemias and hypothyroidism from the major euthyroid hypothyroxinemias.