Cyclosporine whole blood concentrations are routinely monitored to help ensure optimal therapy. Several drugs have been shown to alter cyclosporine disposition by altering hepatic metabolism. Rifampin, a potent enzyme inducer, has been demonstrated to increase cyclosporine clearance. The effects of ansamycin, a chemically related investigational antibiotic, on cyclosporine disposition, have not been previously published. We report a renal transplant patient receiving cyclosporine and consecutive therapy with rifampin and ansamycin. When compared to rifampin, ansamycin's effect on cyclosporine disposition appeared to be delayed and of less magnitude. Further characterization of this interaction is required. Until this issue is resolved, we would advise careful monitoring of cyclosporine blood levels in patients receiving either rifampin or ansamycin in concert with cyclosporine.