Abstract Computerized Chou-Fasman analysis of the secondary structure of human T-cell leukemia viruses (HTLV-I, HTLV-II) and human immunodeficiency virus (HIV) envelope proteins revealed that only one antigenic epitope (amino acids EAL) is shared by the three viruses. A similar antigenic epitope is also found in human and rat brain hormone vasopressin-neuropohysin. If autoantibodies in multiple sclerosis (MS) are made to the epitope EAL, they may cross-react with the envelope proteins of HTVL. It is speculated that in AIDS patients, antibodies to the antigenic epitope EAL of HIV may cross-react with brain vasopressin-neurophysin, leading to a decline in this brain peptide hormone. Thus it is hypothesized that treatment of both MS and AIDS patients with a synthetic polymer containing the amino acids EAL might eliminate the antibodies to vasopressin-neurophysin and thus alleviate some of the clinical symptoms.