Initial studies in renal and recent studies in adult cardiac transplant patients have shown Zenapax(R) to be both efficacious and safe when used in several different dosing schedules. Little data is available regarding pharmacokinetics, safety and appropriate dosing in pediatric heart transplant patients. Yet this ever-increasing group of patients presents a significant challenge for the prevention of primary rejection and the appropriate maintenance of immunosuppression. Induction of long term allograft acceptance through peripheral tolerance has been shown in animal models to be more easily induced in young animals. Once established however, allograft rejection and immunologic responses in the young are quite vigorous. This dichotomy makes young allograft recipients a particularly attractive population for the study of immune modulators targeted at preventing proliferative expansion of alloreactive T cell clones. This is precisely the mode of action of anti-IL2R monoclonal reagents such as Zenapax(R).