Patient entry is now complete in a prospective trial of anti-Tac, a murine IgG2a monoclonal antibody directed against the p55 chain of the human IL-2 receptor, for the prevention of renal allograft rejection. Recipients of primary cadaver allografts were randomized to receive either anti-Tac (20 mg q.d. x 10 days beginning POD 1) plus low-dose CsA (4 mg/kg/day), azathioprine (2 mg/kg/day), and prednisone (30 mg q.d.), or conventional triple therapy with CsA (8 mg/kg/day), azathioprine, and prednisone. Forty patients were entered in each group, with current followup from 6 to 26 months. The results show a significant reduction in early rejection episodes in the anti-Tac-treated patients. During the 10-day treatment, 5 of 40 anti-Tac patients had rejection episodes, compared with 21 of 40 control patients (P less than 0.001). Anti-Tac significantly delayed the time to the first rejection (12.5 +/- 6.3 vs. 7.6 +/- 6.7 days) (P less than 0.05). Despite these effects, there were no differences in either actual or actuarial graft or patient survival between the two groups. Pneumonia, primarily CMV, developed in 5 treated and 4 control patients. In patients with functioning grafts mean serum creatinine at 3 months was 1.8 +/- 0.7 in the anti-Tac group and 2.0 +/- 0.8 in the control group (P = NS); at 12 months the values were 2.3 +/- 1.5 and 1.8 +/- 0.5, respectively (P = NS). The peak expression of IL-2 receptors on circulating T-cells was significantly lower in anti-Tac patients (15.1 +/- 3.6%) than in controls (21.9 +/- 4.5%) (P less than 0.05). Seven of 10 patients tested to date developed antimouse immunoglobulin antibodies, with antiidiotype shown in 6. These antibodies do not preclude subsequent treatment with OKT3. Five patients in this and previous anti-Tac protocols have received OKT3 for acute rejection despite known pretreatment antimouse antibodies, with resolution of rejection in all cases.