The immunosuppressive effect of a monoclonal antibody (moAb), BT563, directed to the alpha-chain of the IL-2R (CD25), was analyzed in a prospective nonrandomized trial and a prospective randomized trial. Primary objectives were evaluation of the incidence of acute rejections and infections; secondary objectives were safety and tolerability of the moAb. A total of 28 patients were enrolled (phase II) to receive 10 mg/day of BT563 (12 days) as immunoprophylaxis in combination with cyclosporine, azathioprine, and low-dose steroids. Subsequently 32 patients were randomly assigned (phase III) to receive BT563 (10 mg/day) for 12 days or ATG (5 mg/kg/day) for 7 days in addition to cyclosporine and low-dose steroids. No side effects of the BT563 treatment were noted. The actuarial survival was 82% at 12 months in the phase II trial and 92% at 12 months in both arms of the phase III trial. There was one acute rejection in the phase II trial. No acute rejections were noted in the BT arm of the phase III trial and 5 acute rejections were treated in the ATG arm. In the phase II trial 7 infectious episodes were observed, while one infection was seen in the BT arm and 7 in the ATG arm of the triple immunosuppression phase III trial. In all patients circulation of coated CD25+ lymphocytes was observed during BT563 treatment; there was no evidence of depletion or modulation of CD25+ cells. Mean serum levels of BT563 ranged from 1.6 to 7.6 microgram/ml throughout the therapy. An antimurine response was seen in 82% (phase II) and 100% (phase III) of the patients. Antirabbit antibodies were found in 56% of the patients treated with ATG. Analysis of the antimurine response specificity revealed in 56% blocking anti-isotypic antibodies and only in 3% of the patients an anti-idiotypic response. The data of the study presented suggest that therapy with an anti IL-2R moAb is at least equal to ATG application according to the incidence of acute rejections and infections.