RS-61443, a morpholinoethyl ester of mycophenolic acid, has been shown to prevent renal allograft rejection in dogs when administered in combination with low-dose cyclosporine and prednisolone. The purpose of this study was to test whether high-dose RS-61443 can reverse acute renal allograft rejection. Mongrel dogs receiving a renal allograft were treated with baseline immunosuppression consisting of RS-61443 10 mg/kg, cyclosporine 5 mg/kg, and prednisolone 0.1 mg/kg. All animals developed acute allograft rejection. Dogs in group I (n = 11) received 14, 7, and 3.5 mg/kg methylprednisolone intravenously on 3 consecutive days after the diagnosis of rejection. Dogs in group II (n = 16) were given RS-61443 80 mg/kg twice daily. After rejection treatment, RS-61443 was increased to 20 mg/kg in all animals; cyclosporine and prednisolone were continued as before. In group I, five of 11 dogs developed uncontrollable rejection; in six of 11 dogs only a temporary reversal occurred. None of the dogs in group I survived for more than 20 days after the diagnosis of rejection. In group II rejection was completely reversed in 14 of 16 dogs (87.5%), resulting in a return of serum creatinine to prerejection levels. Thus, high-dose therapy with RS-61443 can successfully reverse acute kidney allograft rejection in dogs in a high proportion of cases.