The role of polymixin B in endotoxaemia due to obstructive jaundice and mesenteric ischaemia has been examined. Rats made jaundiced by common bile duct ligation were challenged with intragastric endotoxin. Rats given polymixin B infusions had improved survival (11/15) compared with controls (4/14). Mesenteric ischaemia was induced in rats by clipping the mesenteric artery. Limulus assay revealed marked endotoxaemia in controls. Polymixin B infusions reduced the degree of endotoxaemia. Pre-treatment with non-absorbable intestinal antibiotics markedly reduced endotoxaemia. Polymixin B infusions gave no advantage of survival (3/10) compared with controls (2/10), but all animals pre-treated with intestinal antibiotics survived (12/12). The difference in results found between obstructive jaundice and mesenteric ischaemia may be due to different degrees of endotoxaemia. Polymixin B may have a place in the prophylaxis of endotoxaemia in clinical obstructive jaundice.