The monoclonal antibody OKT3, administered for 2 to 3 weeks in a dosage of 0.1 mg/kg, in combination with azathioprine and prednisone, proved more effective than conventional antilymphocyte serum for the prevention of renal transplant rejection. Side effects were less marked with OKT3 than with antilymphocyte serum. Infectious complications, especially viral, occurred with the same incidence in both groups. In the long term, following discontinuation of OKT3 or antilymphocyte serum, incidence of rejection episodes and graft survival rates were identical in both groups. OKT3 in a lower dosage of 0.05 mg/kg was less effective, with several patients developing rejection during treatment, and induced more common and earlier anti-OKT3 immunization.