A teicoplanin fluorescence polarization immunoassay (FPIA) developed by International BioClinical (IBC) was evaluated by using serum samples from patients who had been receiving teicoplanin at Detroit Receiving Hospital (DRH) as part of a clinical investigation. Patient samples collected over a 1-year span were assayed at DRH and at IBC, and the results were compared with those of a standard microbiological assay performed at Merrell Dow Research Institute, Indianapolis, Ind. The FPIA has a rapid turnaround time (circa 20 min), utilizes small sample volumes (less than 100 microliters) and is sensitive and accurate in determining concentrations in the range of 5 to 100 micrograms/ml. The intra-assay and interassay coefficient of variation for controls (7, 35, and 75 micrograms/ml) was less than or equal to 13%. Concentrations greater than 100 micrograms/ml must be diluted prior to the assay, which may introduce additional error in determination. The FPIA compared well with the bioassay (r = 0.901) for 193 clinical samples. The results obtained utilizing the FPIA system were reproducible at two different sites, as illustrated by the high degree of correlation between the results at DRH and IBC (r = 0.92). There was less than 7% interference noted when teicoplanin was assayed in the presence of other antibiotics. Patient samples stored for up to 1 year retained their potency: the mean recovery rate in these samples was 107%. The FPIA should be useful for monitoring and adjusting teicoplanin dosage regimens in patients.