The role of lactulose in preventing endotoxaemia in obstructive jaundice has been investigated. A prospective study was performed on 24 consecutive patients with obstructive jaundice undergoing surgery. Twelve patients were given oral lactulose before operation and were compared with twelve controls. Endotoxaemia was reduced in peroperative portal (P less than 0.05) and postoperative systemic (P less than 0.05) blood samples in the lactulose treated group, and a significant fall (P less than 0.05) occurred in the postoperative 24 h creatinine clearances in controls compared with the lactulose treated group. Results from animal experiments in which oral lactulose reduced endotoxin related mortality in obstructive jaundice (P less than 0.05), and the in vitro demonstration of a direct anti-endotoxic action of lactulose suggest that its beneficial action is due in part to an inactivation of endotoxin.