The LPS induced synthesis of tumor necrosis factor in macrophage cultures, as determined in a fibroblast cytolysis assay was found to be effectively blocked by inhibitors of lipoxygenases. Likewise, the presence of tumor necrosis factor in serum of D-galactosamine sensitized mice after challenge with endotoxin was suppressed by the lipoxygenase inhibitors. Indomethacin, a blocker of cycclooxygenase was neither in vivo nor in vitro effective in the prevention of the endotoxin-induced synthesis of TNF. From LPS-treated macrophages we were able to isolate 13-hydroxylinoleic acid, a lipoxygenase product, which is significantly increased after LPS treatment of the cells, covalently bound to cellular constituents and may, therefore, be possibly involved in the formation of TNF.