FK506 is a novel immunosuppressive agent which is approximately 100 times as potent as cyclosporine in vitro. In this initial trial, 65 renal transplant patients of high complexity received primary FK506 immunosuppression. Overall, graft and patient survival rates are 80% and 98.5%, respectively. A major advantage of FK506 is its potency with relatively few side effects, which has permitted elimination of steroids in 31 (60%) of these patients. Because of these encouraging results, a randomized trial comparing the therapeutic efficacy and toxicity of FK506 and cyclosporine is currently underway at our institution.