This protocol is designed to obtain information on the drug levels, metabolism, and safety of daclizumab (Zenapax(R)) in children and adolescents undergoing cardiac transplantation. In addition to the drug safety and metabolism information, the number and severity of rejection episodes in patients undergoing cardiac transplantation using the standard immunosuppressive drugs plus daclizumab will be compared with patients who have previously undergone cardiac transplantation at the Baylor College of Medicine and received the same standard immunosuppressive drugs without daclizumab.