A semiautomated fluorescence polarization immunoassay (FPIA) for total T3 performed using the TDx analyzer has been evaluated. Precision of the assay at a low level of T3 (< 1 .O ng/mL) is not good with CVs of about 17% observed. Precision at moderate to high levels (1.7-5.6 nglmL) is good (CVs = 11.3-4.1 O/O). Low end imprecision is attributable to a lack of sensitivity, with a reliable detection limit of only about 0.9 nglmL. An average recovery of added T3 of 96% was noted, and the assay is not affected by interference due to lipemia, bilirubinemia, or hemolysis. A methods corn -parison study of the FPIA test and total T3 by RIA yielded a regression equation of Y = 1.08 x -.28, r = 0.95. The TDx FPIA total T3 test is a quick, semiautomated procedure, offering the advantage of a stable, stored calibration curve. It is useful for quantitating elevated levels of total T3 as found in hyperthyroid conditions, and its performance is comparable to or better than RIA procedures for this purpose. However, it cannot be recommended for the evaluation of patients with T3 values below 1.0 nglmL, and RIA tests are still preferable for this analytical range.