Tacrolimus is an immunosuppressant that prolongs the survival of host and transplanted tissue in clinical trials of animal models of liver, kidney, heart, bone marrow, small bowel, pancreas, lung, skin, and cornea. The drug is very promising and has been demonstrated to suppress both humoral and cell-mediated immunity. Although the exact mechanism is unknown, tacrolimus appears to inhibit T-lymphocyte activation. Side effects include tremor, headache, diarrhea, hypertension, nausea, and renal dysfunction.