We report here the evaluation of the Abbott TDx assay with a monoclonal antibody for selectively quantifying cyclosporine (CsA) in whole blood. Over the clinically relevant concentration ranges, results with this assay demonstrated within- and between-run CVs of less than 2.5% and 5%, respectively; sensitivity of 25 micrograms/L; good analytical recovery (100.3%); and linearity with whole-blood specimens. The percentage cross-reactivity of the major CsA metabolites varied from 15.3% for AM9 (M-1), 8.2% for AM1 (M-17), and 3.7% for AM4N (M-21), to less than 3% for the other metabolites tested. Results with the TDx assay (y) correlated well with those by the Sandimmune selective RIA (x; Sandoz) with blood specimens from 44 renal-transplant recipients (n = 44, x= 187.3, y = 198.9, y = 5.49 + 1.03x, r = 0.987). The TDx values were on average 24% higher than those by HPLC (x') with the same patients' specimens (n = 44, x' = 159.9, y = 198.9, y = 15.9 + 1.14x', r = 0.967). We conclude that the Abbott TDx monoclonal antibody assay provides a rapid, precise, and accurate means for quantifying CsA in whole blood.