Children have preliminary testing and evaluation to determine eligibility and health. The dosage schedule varies with the dose. During the course of the study, children are monitored for safety through physical exams and blood tests. They have blood withdrawn to study the response to rCD4 and measure the activity of rCD4 in the body. Children may receive immunization of DPT (diphtheria, pertussis, tetanus) or DT and a polio vaccine to measure their antibody response. If the rCD4 is beneficial, children may continue treatment. The study is conducted in four parts: