A fluorescence polarization immunoassay (FPI) for teicoplanin that uses the TDx Instrument System (Abbott, Irving, Tex.) as an automated analyzer has been developed by Innotron of Oregon Inc. and was evaluated in patients with staphylococcal infections enrolled in a clinical trial of the antibiotic. The assay proved accurate in estimating concentrations of between 5 and 100 mg/liter. The intraassay coefficient of variation was < 7.3%, while the interassay variance was < 11.6% against three commercially prepared standards at known concentrations of approximately 5, 35, and 75 mg/liter. Against routinely prepared standards at 10 concentrations between 5 and 100 mg/liter analyzed in a single run, the coefficient of variance did not exceed 4.3%. Compared with bioassay, the FPI demonstrated good correlation in terms of reliability (r = 0.909) in samples containing teicoplanin only and specificity (r = 0.916) in samples containing both teicoplanin and gentamicin. With a turnaround time of 20 min and with only 50 microliters of serum needed for estimation of the amount of drug in a sample, the FPI described here should provide a useful method of teicoplanin measurement in routine diagnostic laboratories.