Mucinase and beta-glucuronidase enable colon bacteria to degrade protective mucins and recycle glucuronide conjugates of toxins and carcinogens. The response of these bacterial enzymes to dietary fiber was studied in the laboratory rat. Fiber-free basal diet was mixed with guar gum, pectin, carrageenan, or cellulose at levels of 5 and 15%. These diets were fed for 21 days to groups of six male Fischer-344 rats having an average weight of 150 g. Mucinase and beta-glucuronidase activities were assayed in fresh rat feces. Rats fed 15% guar gum or pectin gained significantly (P less than 0.05) less weight than the other rats. Mucinase specific activity was highest in the fiber-free diet group and lowest in the 15% guar gum group. Total daily output of mucinase was highest in rats fed fiber-free diet or cellulose and lower in rats fed more readily fermentable fiber. Specific activity and total output of beta-glucuronidase were highest in rats fed fiber-free diet and significantly lower in those fed 15% fiber diets. These data are consistent with the hypothesis that some kinds of dietary fiber may play a role in the etiology of intestinal disease.