The success of anti-rejection therapy was evaluated in 65 cadaveric renal allografts immunosuppressed with cyclosporin. All first rejection episodes were treated with 0.5 g methylprednisolone given daily for 3 days. Second rejection episodes were treated with either a further identical 3-day course of methylprednisolone or a 10-14-day course of the monoclonal antibody OKT3. The peak creatinine during second rejection episodes was higher in patients treated with OKT3 (481 +/- 54 mumol/l; mean +/-SEM) than in those treated with methylprednisolone (320 +/- 59 mumol/l) (P = 0.036), suggesting more severe rejection in the OKT3 group. Despite this the rejection reversal rate was higher in the OKT3-treated group (100%) when compared to the high-dose methylprednisolone group (64%) (P less than 0.05) and the creatinine at 6 months was less in the OKT3 group (162 +/- 17 mumol/l) than in the methylprednisolone group (230 +/- 22 mumol/l). These data suggest that OKT3 is a very effective treatment for second rejection episodes following renal transplantation and yields improved renal functions in comparison with high-dose steroids.