The fluorescence polarization immunoassay (FPIA) method for determination of cyclosporin in plasma was evaluated and compared with the high-performance liquid chromatography (HPLC) and the radioimmunoassay (RIA) methods. The coefficients of variation for the within-run and between-run precision were <5 and <8%, respectively, for samples ranging in concentration from 50 to 600 ng/ml. Recoveries were determined by adding cyclosporin at concentrations from 25 to 1,000 ng/ml to patient plasma; they were, on average, 98.5%. The calibration curve was stable throughout a 10-week study period. There was no clinically significant interference due to hemolysis, icterus, lipemia, or other commonly used drugs. There was considerable variation of the ratio of the FPIA result to the HPLC result, whereas there was a good correlation between the FPIA and the RIA results (r= 0.975, n = 25,y= 1.2x– 36.4), when evaluated using specimens from renal transplant patients receiving cyclosporin orally. It was concluded that the FPIA is an appropriate, rapid method for patient cyclosporin analysis in plasma and serves as a practical alternative to the RIA.