Many toxic substances are excreted via the bile as chemically inert metabolites. In some cases, however, highly toxic, mutagenic and carcinogenic products can be derived from bile, either directly or after further metabolism by intestinal microflora or intestinal cells. Potential reactive substances may thus reach biliary tract and intestinal cells via this route and may undergo subsequent enterohepatic circulation. The possible role of biliary reactants in carcinogenesis of the biliary tract and intestine is discussed.