Peripheral blood CD8+ T lymphocytes from human immunodeficiency virus (HIV)-infected individuals suppress replication of HIV in peripheral blood mononuclear cells (PBMC). This anti-viral activity appears to be mediated in part by a diffusible factor. Production of this lymphokine varies among infected individuals and may reflect the intrinsic ability of an individual's CD8+ cells to control HIV infection. In some cases in which factor activity is not apparent, contact of the CD8+ cells with infected CD4+ cells can produce for suppression of virus replication. These observations could lead to approaches for enhancing anti-viral responses in HIV-infected individuals.