Cyclosporin A trough blood levels were unusually low during rifampin treatment in a kidney transplant patient. Simultaneously, acute graft rejection occurred. Pharmacokinetic investigation revealed a rapid turnover of cyclosporin A leading to low blood levels. Cessation of rifampin therapy reversed these changes. Rifampin substantially reduces the bioavailability of cyclosporin A and should not be used in transplant recipients on cyclosporin A.