Lymphocytopathic retroviruses have been recovered from the peripheral mononuclear cells of over 160 individuals in all segments of the population known to be at risk for developing AIDS. They have not been isolated from individuals coming from outside the risk groups. Virus has been cultured from patients with AIDS or AIDS-related conditions and from clinically healthy individuals. Some of these AIDS-associated retroviruses (ARV) have been selected to be studied for differences in biologic and molecular features. The results support the role of this virus in AIDS and its associated diseases.