Mice injected with viable Listeria monocytogenes or powdered Corynebacterium parvum and challenged with endotoxin released tumour necrosis factor (TNF) into the blood. However, the rate of mortality from endotoxin shock was high. Administration of the non-steroidal anti-inflammatory drugs acetylsalicylate, indomethacin and phenylbutazone protected the animals against the lethal effect of endotoxin without affecting the ability of animals to produce TNF. The significance of these observations are discussed.