This study was conducted to examine the mechanisms of the anti-colon tumor effect of dietary sericin. Dietary supplementation of 3% sericin reduced colon mucosal lipid peroxide and aberrant crypt foci in 1,2-dimethylhydrazine-treated rats. The colon content from sericin-fed rats had much stronger antioxidant activity compared to that from control rats not receiving sericin. The amino acid composition of undigested proteins in the colon contents from sericin-fed rats was similar to that of sericin ingested. The results suggest that the strong antioxidant activity of undigested sericin in the colon content causes lower oxidative stress and tumorigenesis in the colon.