The anti-OKT3 response was studied in 40 paediatric kidney transplant recipients receiving OTK3 as a prophylactic treatment in association with azathioprine and prednisone. Only 1 patient experienced a reversible acute rejection episode while receiving OKT3. OKT3 induced a rapid disappearance of CD3+ cells, but significant proportions of CD3+ cells reappeared before the end of the treatment in 14 patients. Wide variations in circulating OKT3 levels were observed and in only 50% of patients could stable circulating OKT3 levels be detected until discontinuation of treatment. Anti-OKT3 antibodies detected by the enzyme-linked immunosorbent assay (ELISA) (anti-idiotypic and anti-isotypic antibodies) developed in 91% of patients. Anti-idiotypic antibodies detected by the immunofluorescence inhibition test were found in the sera of 71% of patients, always when high titres of anti-OKT3 antibodies were detected by ELISA. As it has recently been shown that anti-idiotypic antibodies are associated with failure of subsequent OKT3 treatment, we conclude that OKT3 should be restricted to steroid-resistant rejection crises in paediatric patients.