A group of 52 liver transplant patients was prospectively randomized to receive prophylactic immunosuppressive therapy consisting of either Orthoclone OKT3 for 14 days, azathioprine, and steroids (25 patients); or cyclosporine, azathioprine, and steroids (27 patients). The groups were similarly matched for age, diagnosis, and Child's classification. The patients were studied to determine the effect of these two regimens on the incidence of rejection, infection, renal dysfunction, and mortality. Seven rejection episodes, as determined by clinical and histological criteria, occurred in seven of 25 patients (28%) receiving OKT3 compared with 18 episodes in 27 patients (67%) receiving cyclosporine during the first 14 days after transplantation (P less than 0.02). In 20% of the OKT3 patients, CD3+ levels of greater than 10% developed during therapy, and 16% of the patients developed anti-OKT3 antibodies during OKT3 treatment. Five patients were retreated with OKT3 for steroid-resistant acute rejection episodes; all had resolution of the rejection episode. Infectious complications were similar in each group. Renal function, as measured by serum creatinine, was significantly better with OKT3 than with cyclosporine (P less than 0.003) at 14 days. We conclude that prophylactic OKT3 is effective in reducing the number of early rejection episodes after liver transplantation; after 14 days the incidence of rejection is similar; reuse of OKT3 has been successful in liver transplant patients; infectious complications are similar between OKT3 and cyclosporine; and OKT3 preserves renal function better than cyclosporine and is thus indicated in patients with compromised preoperative renal function.