The new Abbott TDx cyclosporine and metabolites fluorescent polarization immunoassay procedure provides a 20-min sample turn-around time, using 50 microliters of sample for the analysis of cyclosporine in whole blood. A precipitation agent and a lysing agent are utilized as a pretreatment step. The range of the whole blood assay is from 0 to 2,000 ng/ml, with a sensitivity of 50 ng/ml. Precision studies at 3 control levels provided coefficients of variation of 2.1-4.8% for both assays. In order to compare this assay with the currently used Sandoz polyclonal radioimmunoassay (RIA) method. 200 whole blood samples were obtained from 20 renal, cardiac, and hepatic transplant recipients. The mean whole blood cyclosporine concentrations for samples above the sensitivity level were as follows: TDx 754 ng/ml (+/- 31) and RIA 619 ng/ml (+/- 22). Blood TDx levels correlated strongly with RIA levels, with a regression coefficient of r = 0.915. This new assay provides reliable blood cyclosporine concentrations that correlate well with RIA measurements. This assay offers rapid sample turn-around times, making same-day results for outpatient drug monitoring possible.