A competitive binding assay for serum thyroxine has been automated, with the use of small, reusable Sephadex columns to separate thyroxine from endogenous thyroid-binding globulin and later to separate the bound and free thyroxine. Sixty samples an hour are run through columns, which are held in an aluminum turntable rotated by a fraction collector motor. Reagents and samples are fed to the columns by a proportioning pump. Waste eluates are collected and drained to the sink by a Teflon tray positioned between the columns and counting tubes, also held by the turntable. A cut out area in the tray allows one to collect the bound fraction, which can then be counted in a scintillation counter. Values are calculated and printed by a desk calculator interfaced with the scintillation counter. The "day-to-day" CV for this method is 5%. The accuracy is satisfactory when tested by comparison with other methods, by recoveries, and by linearity of dilutions.