In a prospective clinical study we tested the immunosuppressive properties and toxicity of low-dose OKT3 induction therapy in renal transplant recipients. 50 consecutive renal transplant recipients were alternatingly assigned to low-dose OKT3 induction or prednisolone/cyclosporin. Low-dose OKT3 induction treatment consisted of 0.5 mg OKT3 twice daily for 10 days, initially combined with azathioprine and prednisolone maintenance immunosuppression that was converted to prednisolone/cyclosporin at the end of the course. During a 15-29-month follow-up period, low-dose OKT3 induction therapy was found to reduce significantly the incidence of acute rejections, as compared to the usual prednisolone/cyclosporin maintenance immunosuppression (21 versus 52%, P = 0.02). There also was a tendency towards an improved graft function after low-dose OKT3, although no significance was reached. Furthermore, compared to a historical control group of renal transplant patients in whom acute rejection was treated with 5 mg OKT3 daily, low-dose OKT3 appeared to cause fewer side-effects. We conclude that low-dose OKT3 induction therapy is superior to prednisolone/cyclosporin in preventing acute rejection after renal transplantation and that it is better tolerated than conventional OKT3 treatment.