Tumor necrosis factor (TNF) is liberated by macrophages in response to infection. It has different biological effects, namely cytotoxicity on some tumor cell lines, anorexia, hyperlipidemia, and decreases in protein synthesis. It favors immunological responses and produces necrosis of certain tumors. The mechanisms of action of TNF are not well understood. Because the gene is found in all vertebrates, it seems that TNF plays an essential role, possibly a defensive one, even though large doses have severe effects, such as shock, fever and death. Endotoxic shock is mediated by this endotoxin with the help of IL-1. Other effects of TNF seem to be mediated by the metabolites of arachidonic acid.