Rejection episodes in renal allograft recipients are usually efficiently treated with high doses of intravenous methylprednisolone. Rejection therapy with OKT3 is often reserved for steroid-resistant episodes. However, the optimal dose of OKT3 in the treatment of steroid-resistant rejection is not known. Therefore, we randomized renal transplant recipients with steroid-resistant rejection to treatment with a standard daily intravenous dose of either 5 mg of OKT3 (n=15) or 2.5 mg of OKT3 (n=15) for 10 days. Circulating T cells (measured as CD2+ cells) were adequately and equally depleted in the two groups. Three grafts were lost due to rejection within the first 3 months following OKT3 administration, one in the 2.5 mg OKT3 group and two in the 5 mg OKT3 group. Two nonimmunologic graft losses occurred in the 2.5 mg OKT3 group. Median serum creatinine values were not different between the two groups, neither at the start (median values: 200 micormol/L in the 5 mg OKT3 group vs. 188 micromol/L in the 2.5 mg group) nor immediately after OKT3 rescue therapy (202 micromol/L vs. 185 micromol/L, respectively). Eight cytomegalovirus infections occurred in each group. Two re-rejection episodes occurred in the 5 mg OKT3 group and one occurred in the 2.5 mg OKT3 group. All responded to treatment. Function of the remaining grafts estimated by serum creatinine after a mean long-term follow-up of 18 months (range, 6-36 months) revealed no differences (185 micromol/L in the 5 mg OKT3 group vs. 170 micromol/L in the 2.5 mg OKT3 group). We conclude that OKT3 treatment of steroid-resistant rejections in renal transplant recipients is equally effective in daily doses of 2.5 mg and 5 mg with respect to reversal rate and long-term outcome.