The objective of this study is to assess the pharmocokinetics, safety, and tolerability of FK 506 and mycophenolate mofetil when used in combination instable renal transplant recipients. 18 stable renal transplant recipients will be allowed to participate in this study. Six patients will be assigned to each treatment group (mycophenolate mofetil 500 mg BID, mycophenolate mofetil 750 mg BID and mycophenolate mofetil 1000 mg BID).