Since it has been demonstrated that a high level of fat is a dietary factor in the etiology of colon cancer, the effect of carrageenan, a polysaccharide extracted from the red seaweeds, on 1, 2-dimethylhydra-zine-induced colonic tumors in rats fed a semipurified control diet containing an ordinary level of fat was studied. Nevertheless, the enhancing effect of carrageenan on colonic tumors was observed. The rats fed a carrageenan diet had approximately twice the fecal weight compared to the rats fed a control diet. While no significant differences were found in β-glucuronidase activities in colonic mucosa, liver or plasma in the car-rageenan-fed rats and controls, the activity in feces was significantly lower in the carrageenan-fed rats. At least, no β-glucuronidase activity seemed to be related to the tumor-enhancing effect of carrageenan.