The cell biology studies have been focused on the role of human T-lymphotropic retroviruses on human T-cell malignancies and acquired immune deficiency syndrome (AIDS). More than 100 isolates of HTLV-I have been obtained from patients with adult T-cell leukemia/lymphoma, and over 200 isolates have been obtained from patients with AIDS and ARC. HTLV-I has been shown to be a transforming virus whereas HTLV-III is cytopathic. Both HTLV-I and HTLV-III have specificity for OKT4 positive T helper cells. HTLV-III has been transmitted to a cell line which is productively infected with HTLV-III thus allowing the production of large quantities of virus for cell biology, molecular biological and biochemical studies, and the development of an ELISA test for screening of blood. HTLV-III has also been isolated from saliva, semen, and tears from AIDS or ARC patients. HTLV-III isolates obtained from different patients show some genetic variations in the envelope region. HTLV-III from a Haitian AIDS patient has been found to be the most divergent compared with other HTLV-III isolates. The virus probably attaches to the T cells through the OKT4 receptor since treatment of the virus with anti-T4 blocks infection of the recipient cells. Drugs that block HTLV-III replication are being tested in in vitro systems. Preliminary studies indicate foscarnet, AL 721, and D-penicillamine may be useful in treatment of patients with AIDS or ARC. Studies in chimpanzees indicate the development of antibodies against HTLV-III antigens and viremia in these animals on inoculation with HTLV-III. Vaccine studies indicate the development of neutralizing antibodies against the virus envelope, and more recently antibody against thymosin Alpha-1 was found to block HTLV-III replication in cell culture. Thymosin Alpha-1 has 50% mohology with HTLV-III p17, suggesting the possible use of a synthetic peptide from the conserved region to be an effective vaccine for AIDS.