We prospectively studied the use of prophylactic Minnesota antilymphocyte globulin vs. OKT3 in kidney transplant recipients. Between 7/1/87 and 9/1/90, 138 adult kidney and 35 kidney-pancreas recipients were randomized after stratification for age (18-49 vs. greater than or equal to 50), diabetes (diabetic vs. nondiabetic), transplant number (1 vs. greater than 1) and, for retransplants, the length of survival of the first graft (less than 1 year vs. greater than or equal to 1 year), and then randomized to receive 7 days of either MALG (20 mg/kg/day) or OKT3 (5 mg/day). Immunosuppression was otherwise identical in both groups; prednisone and azathioprine started on the day of surgery, and cyclosporine started on postoperative day 6. Minimum follow-up was 9 months. There was no difference in one- and two-year actuarial patient or graft survival rates, incidence of rejection, or serum creatinine level. MALG was associated with a higher incidence of cytomegalovirus; it was statistically significant in the subgroup of CMV seronegative recipients of kidneys from seropositive donors (P less than .05). OKT3 was more expensive and was associated with significantly more side effects: fever (P less than .0001), dyspnea (P = .04), and acute respiratory distress syndrome (ARDS) (P = .02).