Since the murine monoclonal antibody OKT3 reacts with human T cells and blocks their function, we explored its effectiveness in treating T-cell-mediated rejection of renal allografts. In a prospective randomized multicenter trial, 123 patients undergoing acute rejection of cadaveric renal transplants were treated either with OKT3 daily for a mean of 14 days, with concomitant lowering of the dosage of other immunosuppressive drugs (63 patients), or with conventional high-dose steroids (60 patients). OKT3 reversed 94 per cent of the rejections--a figure that was significantly better (P = 0.009) than the 75 per cent reversal rate obtained with conventional steroid treatment. This superior reversal rate with OKT3 was reflected in an improved one-year graft survival of 62 per cent for the OKT3-treated group, as compared with 45 per cent for the steroid-treated group (P = 0.029), in patients who were all selected by virtue of having had acute rejection. We conclude that treatment with OKT3 (with concomitant lowering of the dosage of other immunosuppressive drugs) is an effective approach for acute renal-allograft rejection.