Monocytes express cytotoxic factors of the tumour necrosis factor (TNF) ligand superfamily, including TNF and Fas ligand, both on the cell surface and in secreted form. In this report, we show that human monocytes and monocyte-derived macrophages stimulated with lipopolysaccharide (LPS) express APO2 ligand (APO2L, TRAIL), a recently discovered cytotoxic member of the TNF ligand superfamily. LPS increased the transcription of APO2L mRNA in monocytes and macrophages. Flow cytometric analysis showed low surface and high intracellular levels of APO2L, and LPS increased the expression of both. In addition, LPS increased the monocyte- and macrophage-mediated cytotoxicity against the APO2L-sensitive Jurkat and RPMI-8226 cells. Addition of the APO2L-binding decoy receptor 1 (DcR1)-Fc fusion protein inhibited the cytotoxicity by 30-70%. LPS also stimulated the release of soluble APO2L from the monocytes and macrophages. Monocytic phagocytosis of target cells was increased by LPS and partially inhibited by DcR1-Fc. Taken together, these data demonstrate a novel mechanism of cytotoxicity mediated by LPS-activated human monocytes and macrophages.