OKT3 has become one of the more effective antirejection therapies for patients receiving kidney transplants. However, its usefulness is diminished or blocked by the development of antimouse/anti-OKT3 antibodies. We evaluated 17 children receiving OKT3 for steroid-resistant acute rejection for the development and persistence of antibodies after therapy. OKT3 was successful in reversing acute rejection in 14 of 17 patients. Eight children developed antimouse antibodies, 7 at a low titer (1:100). The retesting of all children 6 months later showed no detectable antibodies. Children develop anti-OKT3 antibodies at a rate similar to adults and with time lose detectable levels which may have significance if a subsequent course of OKT3 is needed.