The Technicon Immuno 1 free thyroxine (fT4) assay, a modified two-step procedure that is fully automated on a random access analyzer, was evaluated-at two clinical sites. The method had excellent precision and correlated well overall with three other estimates of free thyroxine: free thyroxine index (FTI) measured on the Immuno 1; the Abbott IMx fT4 assay (Abbott Park, IL); and the Clinical Assays two-step manual fT4 assay. Using a combination of thyrotropin and FTI assay results as a "gold standard" for defining thyroid status, the Immuno 1 fT4 method had a sensitivity of 100% and specificity of 98.3% for hyperthyroidism, versus 93.8% and 99.3%, respectively, for hypothyroidism. In conclusion, the Immuno 1 fT4 assay is useful in screening for thyroid disease.