Tacrolimus is being increasingly used as an immunosuppressive agent in transplant recipients. The drug is metabolized by the cytochrome P-450 3A system, thus antimicrobials that inhibit or induce these enzymes can alter levels of tacrolimus in the bloodstream. Tacrolimus is potentially nephrotoxic and neurotoxic; the likelihood of toxicity rises as blood levels of the drug increase. Increased tacrolimus levels and subsequent toxicity have been produced by a number of antimicrobial agents that inhibit the cytochrome P-450 3A system. Conversely, drugs with the potential to induce the cytochrome P-450 3A system can reduce the levels of tacrolimus in the blood, leading to increased risk of acute rejection in transplant recipients. Antimicrobial agents that can have adverse effects on renal function may add to the nephrotoxicity of tacrolimus. The potential for drug interactions should be reviewed before antimicrobial agents are prescribed to patients treated with tacrolimus.