Japanese kelp (kombu) includes a xanthophy, fucoxanthin. In the present study, the chemopreventive activity against the formation of aberrant crypt foci (ACF, a preneoplastic marker for colon cancer) was examined. Four groups of mice were housed with drinking water containing 5 mg/ml of a kombu ethanol extract, and 0.05 mg/ml and 0.1 mg/ml of purified fucoxanthin, and with water alone as the positive control. They were subcutaneously injected with 10 mg/kg b.w. of azoxymethane (AOM) on the 1^st and 8^th days. Another group served as a negative control without kombu and AOM. The positive control produced a substantial number of ACF by the 28^th day. Significant reductions were found in the frequency of ACF: 36% for the kombu extract, 30% for 0.05 mg/ml fucoxanthin and 35% for 0.1 mg/ml fucoxanthin. In the liver of these mice, the kombu extract raised the level of activity of glutathione S-transferase (GST) and quinone reductase, and stimulated the antioxidant responsive element (ARE). Fucoxanthin enhanced the GST activity but did not affect the ARE. Thus, one of the kombu ingredients, fucoxanthin, can suppress the ACF formation induced by the colon carcinogen AOM, and other unknown ingredients can induce the ARE activation.