In order to understand the function of carrageenan, an indigestible polysaccharide, as a promoter of colonic tumors induced by 1,2-dimethylhydrazine (DMH), molecular weight distribution of fecal carrageenan and amounts of fecal bile acids in rats given carrageenan and DMH treatment were examined. Gel filtration pattern on Sephacryl S-300 of fecal carrageenan was very similar to that of feeding carrageenan, and carrageenan ingested was quantitatively excreted in feces. Hexafluoroisopropyl ester-trifluoroacetyl derivatives of fecal bile acids were analyzed by gas chromatography on QF-1. Although there was a decreased concentration of deoxycholic acid and total bile acids in carrageenan-fed rats compared to control rats, no difference in the daily output was found because carrageenan ingestion increases fecal output. Significant increased concentration and daily output of lithocholic acid, a tumor-promoter, by feeding carrageenan were found. Thus, it was suggested that the promoting effect of carrageenan on colon tumorigenesis by DMH may be mediated by increased excretion of lithocholic acid and may not participate in degradation of carrageenan ingested.