Lymphokine-activated killer (LAK) cells generated from perforin knockout mice possess significantly reduced cytotoxicity against a panel of tumor target cell lines, with some tumor cells being lysed exclusively by the perforin pathway. LAK cells are also capable of Fas ligand-mediated cytotoxicity. LAK cells generated from mice deficient in both perforin and Fas ligand (PKO/gld) were not cytolytic in short term cytotoxicity assays, demonstrating that perforin and Fas ligand are required for acute target cell lysis. However, PKO/gld LAK cells were cytotoxic in long term cytotoxicity assays against TNF-sensitive tumor lines, and this cytotoxicity was completely inhibited by neutralizing TNF Abs. This potent TNF cytotoxicity has not been fully appreciated previously because of the presence of dominant-acting perforin and Fas ligand in acute tumor cell lysis. TNF-based cytotoxicity by PKO/gld LAK was both soluble and membrane bound, and both forms of TNF were constitutively expressed. Thus, LAK cells are armed with at least three cytotoxic molecules: perforin, Fas ligand, and TNF.