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 100 isolates have also been obtained from patients with AIDS and pre-AIDS. 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. HTLV-III has also been isolated from saliva or semen from AIDS or pre-AIDS patients. HTLV-III isolates obtained from different patients show some genetic variations. An 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 receipient cells. Drugs that block HTLV-III replication are being tested in in vitro system. Preliminary studies indicate foscarnet, cis platinum nd lapachone derivates may be useful in treatment of patients with AIDS or pre-AIDS.