Studies have shown that AIDS is caused by a retrovirus. This virus works by inactivating or destroying human CD4 cells (which are part of the human immune system). This in turn leads to the observed immunologic defects and related illnesses, including HIV encephalopathy (disease of the brain). One method of preventing AIDS is to prevent HIV from entering the cell. HIV binds to the receptor CD4 site. Peptide T also binds to this site, and thus by competing for that site, can block the binding of the virus to its receptor. Preliminary animal and human studies indicate that peptide T is safe at the doses selected for this trial.