The effect of dietary fat, starch and cellulose on individual and total fecal bile acids was studied in mice after 4 wk of feeding diets containing different levels of fat (5 and 29%), starch (3, 36 and 57-65%) and cellulose (2 and 10%). Diet affected the fecal concentration of deoxycholic acid, beta-muricholic acid and total bile acids. Increasing dietary fat from 5 to 29% significantly increased the level of deoxycholic acid and total bile acids. An increase in dietary cellulose from 2 to 10% significantly decreased the level of deoxycholic acid, beta-muricholic acid and total bile acids. High levels of dietary starch (36 and 57-65%) did not significantly affect the excretion of deoxycholic, beta-muricholic and total fecal bile acids. Starch was able to bind bile acids in vitro and to affect the level of fecal free bile acids. In high fat diets, the level of free bile acids was lower in the feces of animals fed 36% starch diets than in those fed 3% starch diets. This reduction of free bile acids was accompanied by a reduction in colon proliferative activity. We suggest that free, rather than total, bile acids are the effective damaging agents for colon mucosa, and may represent a risk factor in the development of cancer.