Individuals infected with HIV have elevated numbers of total and activated CD8+ lymphocytes in peripheral blood. CD8+ lymphocytes from HIV-infected individuals have been shown to mediate non-human histocompatibility-linked antigen (HLA)-restricted suppression of viral replication, HLA-restricted killing of cells expressing HIV antigens, and killing of uninfected lymphocytes. We studied CD8+ T lymphocytes that lysed autologous CD4+ lymphocytes, hetcrologous CD4+ lymphocytes from HIV-infected individuals and uninfected CD4+ lymphocytes. Killing in all cases required T cell receptor (TCR)-mediated recognition or triggering. However, these CD8 cytotoxic T lymphocytes (CTL) killed HLA class I mismatched CD4+ lymphocytes and CD4+ lymphocytes treated with a MoAb against HLA-A, B and C antigens (PA2.6) which blocks HLA class I-restricted killing. HLA class H-negativc CD4+ T lymphoma cells (CEM.NKR) were also killed by anti-CD3 inhibited CTL. Stimulation of peripheral blood lymphocytes (PBL) from HIV-infected individuals, but not uninfected controls, with concanavalin A (Con A) and IL-2, induced non-HLA-restricted TCR αβ+, CD8+ CTL which lysed CD4+ lymphocytes. Activation of CD4+ lymphocytes increased their susceptibility to CD8+ CTL-mediated lysis. In HIV infection, a population of non-HLA-restricted CTL which lyse activated CD4+ lymphocytes is expanded. The expansion of CTL with unusual characteristics is interesting, because the stimulus for this expansion is unknown. CTL which recognize activated CD4+ cells could play a role in immune regulation and the pathogenesis of A IDS.