We have identified an individual whose thyroxine (T4) concentration was undetectable with Abbott's fluorescence polarization immunoassay (FPIA) but within the reference range by radioimmunoassay or EMIT(TM) (Syva). The patient's thyrotropin, triiodothyronine, and T-uptake values were within the normal range. The T4 concentration measured by FPIA increased to normal when the immunoglobulin fraction was selectively removed from the serum. When the patient's immunoglobulin fraction was added to normal serum, the T4 content of the normal serum measured by FPIA became falsely low. The patient's antibody interfered with the T4 FPIA by binding to the fluorescein-T4 conjugate. The T-uptake was less affected by the patient's serum because of the low affinity of the patient's antibody to fluorescein-T4 (K = 3.5 x 108 L/mol). The patient's immunoglobulin bound preferentially to fluorescein- T4, in comparison with binding to fluorescein or T4 alone. We conclude that the patient's immunoglobulin bound to an epitope unique to the fluorescein- conjugated T4.