A number of antimicrobial agents may have untoward interactions with cyclosporine. Concurrent use of ketoconazole or erythromycin results in increased serum cyclosporine levels, with associated potential nephrotoxicity or neurotoxicity. Rifampin and perhaps nafcillin induce cytochrome P-450-dependent isoenzyme metabolism of cyclosporine. The resultant decreased bioavailability of cyclosporine may precipitate graft rejection. The aminoglycosides, trimethoprim-sulfamethoxazole, and amphotericin B have been associated with increased nephrotoxicity, independent of the levels of cyclosporine or antibiotic present. Anticipation of potential antibiotic interactions and appropriate monitoring of levels of cyclosporine and antibiotics and of renal function are critical to the avoidance of reversible causes of graft failure or organ toxicity.