We have investigated the ability of different cells present in murine tumors to induce apoptosis of activated CD8+ T cells in vitro. Tumor cells do not induce apoptosis of T cells; however, macrophages that infiltrate tumors are potent inducers of apoptosis. Tumor macrophages express cell surface-associated TNF, TNF type I (CD120a) and II (CD120b) receptors, and, upon contact with T cells which induces release of IFN-γ from T cells, secrete nitric oxide. Killing of T cells in vitro is blocked by Abs to IFN-γ, TNF, CD120a, or CD120b, or N-methyl-l-arginine. In concert with that finding, tumor macrophages isolated from either TNF type I or type II receptor−/− mice are not proapoptotic and do not produce nitric oxide upon contact with activated T cells. Control macrophages do not express TNF receptors or release nitric oxide. Tumor cells or tumor-derived macrophages do not express FasL, and blocking Abs to either Fas or FasL have no effect on macrophage-mediated T cell killing. These results demonstrate that macrophages which infiltrate tumors are highly proapoptotic and may be responsible for elimination of activated antitumor T cells within the tumor bed.