We carried out a randomized prospective trial to compare OKT3 (5 mg/d, 51 patients) with ATG-Fresenius (ATG-F, 4 mg/kg/d, 53 patients) for induction therapy after renal transplantation, concerning side effects, rejection, and infection incidence within a one year follow-up period. Concomitant immunosuppression included azathioprine/steroids from day 0 and cyclosporine A from day 4. OKT3 patients experienced significantly more and more-severe side effects, particularly pyrexia, headache, and pulmonary fluid overload. One-year graft survival was excellent in the ATG-F group (91%), but only 78% in the OKT3 group (P < 0.05) due to a series of rejections that occurred beyond day 100; patient survival (96% and 92%) was similar in both groups. OKT3-treated patients experienced more biopsy-proven rejections (0.6 +/- 0.1/pt.) than ATG-F patients (0.3 +/- 0.1, P < 0.05), and there was a similar, albeit not significant trend in clinical rejections (OKT3: 1.1 +/- 0.2/pt.; ATG-F: 0.8 +/- 0.1/pt.). Infections were more common in the OKT3 group (OKT3: 3.2 +/- 0.3, ATG-F: 2.0 +/- 0.2, P < 0.05), although this was entirely attributable to "minor" infections. On days 1 through 6, CD3 counts were more profoundly depressed with OKT3 therapy. Beyond day 10, however, CD3 counts were lower in the ATG-F group, as were CD2 counts, CD4 counts, and the CD4/CD8 ratio, suggesting a more prolonged immunosuppressive effect of ATG-F. Sensitization occurred more frequently with OKT3 (31%) than with ATG-F (10%), but was usually irrelevant, except in two patients (one in each group), whose grafts were lost because of immunization against OKT3 and ATG-F, respectively. In conclusion, a 7-day induction therapy with OKT3 does not improve outcome or diminish immunological graft loss when compared with ATG-F, but is associated with more rejections, infections, and side effects. ATG-F appears to be preferable for induction immunosuppression after renal transplantation.