CD8-positive T cells are thought to play an important role in the control of infection by human immunodeficiency virus (HIV) as a result of their cytotoxic activity and by releasing soluble factors(1,2) in AIDS patients, the absolute number bf CD8(+) T lymphocytes is decreased in peripheral blood(3,4) and their turnover rate Is increased, suggesting that there is more cell renewal and cell death occurring(5). Anti-retroviral therapy raises CD8(+) T-cell counts in HIV-infected patients(6-8). Here we report that the death rate of CD8(+) T cells by apoptosis increased markedly during HIV infection of peripheral blood mononuclear cells in vitro. Apoptosis is induced in a dose-dependent manner by recombinant envelope glycoprotein gp120 from HIV strain X4, or by stromal-derived factor-1 (SDF-1), the physiological ligand of the chemokine receptor CXCR4. Apoptosis is mediated by the interaction between tumour-necrosis factor-alpha bound to the membrane of macrophages (mbTNF) and a receptor on CD8(+) T cells (TNF-receptor II, or TNFRII). The expression of both of these cell surface proteins is upregulated by HIV infection or by treatment with recombinant gp120 or SDF-1. Apoptosis of CD8(+) T cells isolated from HIV-infected patients is also mediated by macrophages through the interaction between mbTNF and TNFRII. These results indicate that the increased turnover of CD8(+) T cells in HIV-infected subjects is mediated by the HIV envelope protein through the CXCR4 chemokine receptor.