It has been reported that lactulose can reduce biliary secondary bile acid (deoxycholate) composition and by this action can decrease biliary cholesterol saturation in patients with supersaturated bile. We tried to confirm this by feeding 60 g of lactulose per day during a period of 12 weeks to 8 healthy females with colonic adenomatous polyps and slightly elevated biliary cholesterol saturation. Although a 38% reduction of biliary deoxycholate was noted--38% after 4 weeks, and 32% after 12 weeks of lactulose feeding (P less than 0.01)--at the expense of a rise in both primary bile acids, cholate and chenodeoxycholate, no significant change in biliary cholesterol saturation was found after 4 nor after 12 weeks of lactulose feeding. These results do not support the concept that colonic bacterial metabolites of bile acids play a major role in the production of supersaturated bile and cholesterol gallstone formation.