The area under the blood concentration vs. time curves of cyclosporine (24 hr) were determined nonspecifically by the polyclonal RIA, specifically by a monoclonal RIA and by HPLC after an oral and an i.v. dose of CsA in 10 renal transplant patients. The mean blood concentrations determined by monoclonal RIA were 10–20% higher than those measured by HPLC, whereas the concentrations assessed by polyclonal RIA were >100% higher than those determined by HPLC. As a corollary, the pharmacokinetic parameters (clearance, volume of distribution, and systemic availability) differed when the results from the 3 methods were compared. The RIA/ HPLC concentration ratio of CsA was higher after oral than after i.v. dosing when RIA measurements were performed by the polyclonal but not by the monoclonal RIA. These ratios changed continuously during the first 12 hr after the administration when the polyclonal but not when the monoclonal RIA was used. In conclusion, blood concentrations assessed by the 3 methods are not identical, and when compared with the polyclonal RIA the monoclonal RIA exhibits 3 advantages: (1) much less crossreactivity with metabolites; (2) a constant RIA/ HPLC concentration ratio after the third hr after administration of CsA; and (3) a RIA/HPLC concentration ratio that is independent of the route of administration.