Tumour necrosis factor (tumour necrosis factor-alpha/cachectin) plays a critical role in certain physiological defensive responses but causes severe damage to the host organism when produced in excess. There are two forms of tumour necrosis factor, a type II membrane protein of relative molecular mass 26,000 (26K) and a soluble, 17K form generated from the cell-bound protein by proteolytic cleavage. The two forms of tumour necrosis factor and lymphotoxin-alpha (tumour necrosis factor-beta/lymphotoxin), a related protein, have similar but apparently not identical biological activities. A therapeutic agent which inhibited the release of tumour necrosis factor, but did not reduce the cell-associated activity or the level of lymphotoxin-alpha, might preserve the benefits of these cytokines while preventing tumour necrosis factor-induced damage. Here we describe a potent inhibitor of tumour necrosis factor processing and report that it protects mice from a lethal dose of endotoxin.