Tumor necrosis factor (TNF), a potent cytokine produced mainly by activated macrophages, has numerous biological functions, including hemorrhagic necrosis of transplanted tumors, cytotoxicity, an important role in endotoxic shock and in inflammatory, immunoregulatory, proliferative, and antiviral responses (reviewed in Refs 1-4). A related molecule, lymphotoxin, produced by activated lymphocytes, shares many of these properties. There are receptors for TNF and lymphotoxin on the majority of mouse and human cell lines 5-s. Two distinct TNF receptor subtypes were identified by crosslinking radiolabelled TNF in myeloid and epithelial cells 9. These two proteins, which had distinct molecular masses (55 kDa and 75 kDa) I, proteolytic fingerprints, immunoreactivity and glycosylation patterns, are the subject of this brief review.