The focus of progress in transplantation immuno-suppression is to achieve more specific immunosup-pression with monoclonal antibodies. We have already shown that the efficacy of 33B3.1, a rat monoclonal Ig2A directed against the human IL-2 receptor, was similar to that of rabbit antithymocyte globulin in the prevention of acute rejection in first kidney transplants. A similar comparative analysis has been made in 40-sec renal transplants. ATG (1 mg/kg/day) or 33B3.1 (10 mg/day) was administered during the first 10 days postgrafting in association with corticosteroids and azathioprine. Cyclosporine was introduced on day 9 and azathioprine/CsA constituted the patient's maintenance treatment after day 45. Rejection treatment consisted of equine antilymphocyte globulin in both cases and of steroid boluses when patients were under Cyclosporine. One patient in each group died. Graft survival was 90%, 85%, and 79% in the ATG group (n=20) and 100%, 89%, and 89% in the 33B3.1 group (n=20) at 3,12, and 24 months, respectively. Of the ATG group patients, 45% and 40% in the 33B3.1 group had at least one rejection episode, half the episodes in the MoAb cohort occurring under 33B3.1, vs. none in the ATG group. Transplant function was similar in both groups. Viral infections appeared to be more frequent with ATG (60%) than with 33B3.1 (12%), with CMV accounting for half of these in the ATG group, and none in the MoAb group. Tolerance of both agents was good. Of the 33B3.1 recipients, 70% developed anti-33B3.1 antibodies. From these data, we conclude that this anti-IL-2 receptor MoAb seems less effective than rabbit ATG as induction treatment in second kidney transplant patients.