The rationale for the use and effects of 8-anilino-1-naphthalenesulfonic acid in the radioimmunoassay of triiodothyronine is discussed. Historical background for the use of fluorescent probes, such as 8-anilino-1-naphthalenesulfonic acid, to study protein sites is outlined. By using 8-anilino-1-naphthalenesulfonic acid to block the binding of triiodothyronine to thyroxine binding globulin, it is possible to design a sensitive and precise radioimmunoassay for measurement of triiodothyronine in unextracted serum samples. Recovery of triiodothyronine added to serum was excellent, and hyperthyroid serum assayed at various dilutions produced a curve that was virtually identical with the standard curve. Standard curves are prepared either in triiodo-/tetraiodothyronine-free serum or human serum albumin to maintain the same milieu between standards and serum samples. Triiodothyronine values by radioimmunoassay ranged from 80 to 250 ng/100 ml in 243 euthyroid subjects. In patients with thyroid disease, the serum triiodothyronine concentrations were as follows: hypothyroid (n = 27), 1000 ng/100 ml. Twenty-two normal pregnant women had triiodothyronine serum concentrations ranging from 174 to 410 ng/100 ml. To assess a patient's thyroid status properly, serum triiodothyronine measurements are required. The triiodothyronine assay appears to be a sensitive index, particularly in the evaluation of thyroid hyperfunction.