The effect of the 4-quinolone antimicrobial agent ciprofloxacin on the concentration in plasma and the pharmacokinetics of the immunosuppressive agent cyclosporine was studied in 10 bone marrow transplant recipients. There were no statistically or clinically significant changes in cyclosporine trough concentrations or areas under the concentration-time curve following oral doses of 500 mg of ciprofloxacin every 12 h for 4 days. The data suggest a lack of relevant pharmacokinetic interaction of ciprofloxacin with cyclosporine. There was no indication of an enhanced nephrotoxicity for this drug combination.