Tumor Necrosis Factor Alpha (TNF alpha) is a protein secreted by lipopolysaccharide stimulated macrophages，and causes tumor necrosis when injected into tumour bearing mice. TNF alpha is believed to mediate pathogenic shock and tissue injury associated with endotoxemia. TNF alpha exists as a multimer of two，three，or five noncovalently linked units，but shows a single 17 kDa band following SDS PAGE under non reducing conditions. TNF alpha is closely related to the 25 kDa protein Tumour Necrosis Factor beta (lymphotoxin)，sharing the same receptors and cellular actions. TNF alpha causes cytolysis or cytostasis of certain transformed cells，being synergistic with interferon gamma in its cytotoxicity. Although it has little effect on many cultured normal human cells，TNF alpha appears to be directly toxic to vascular endothelial cells. Other actions of TNF alpha include stimulating growth of human fibroblasts and other cell lines，activating polymorphonuclear neutrophils and osteoclasts，and induction