Cytochrome P-450 3A metabolizes CsA into several metabolites with very limited pharmacological activity and toxicity. In 40 liver donors, the relative concentrations of P-450 3A was assessed by immunoblot analysis using a specific monoclonal antibody exhibited a 10-fold variation (mean = 92±50 arbitrary units [AU]/mg) in levels. Problems consequent upon CsA usage (nephrotoxicity, neurotoxicity, and hypertension) occurred in 8 of 34 transplant recipients in the immediate postoperative period, and in these 8 patients the problems were always associated with low graft P-4503A levels (mean = 52±19 A.U./mg, P=0.0003, cf. patients with no toxicity). Four of these patients had CsA levels that were also high, but 4 had CsA levels in the therapeutic range. Episodes of early graft rejection were related to higher P-450 3A levels (mean = 110±24 A.U./mg). Cytochrome P-450 3A levels were also found to be inversely related to CsA whole blood levels (assessed with a specific antibody 12 hr after the intravenous infusion of CsA at 1 mg/kg in the recipients (P< 0.02). Cytochrome P-450 3A can provide information that should allow individualized immunosuppression with CsA maintaining therapeutic levels but avoiding toxicity in susceptible individuals.