A high-molecular-weight fraction after removal of water-soluble peptides from proteinase-treated soybean protein isolate (referred to as HMF) was examined for its effect on preneoplastic lesions in the rat colon. For this purpose, male Fisher-344 rats 7 wk old were divided into 8 groups (n = 5), of which 6 groups received 3 injections of azoxymethane (AOM, 15 mg/kg of body weight) for 3 wk once a week, while all were fed HMF or casein diets supplemented with or without deoxycholic acid (DCA) over a period of 4 wk. Two groups of AOM-treated rats were allowed free access to HMF or casein diets without supplemental DCA, respectively, while the others were pair-fed so as to be well matched in their food intake. There were no significant differences in growth parameters among the pair-fed groups. Feeding HMF diets raised fecal lipid and acidic steroid excretions to a greater extent than feeding casein diets, secondary bile acids being conspicuous among acidic steroids in the excreta irrespective of the presence or absence of DCA supplementation. As a result of observation for colonic aberrant crypt foci (ACF), the intake of HMF proved to reverse the reduction of ACF appearance by DCA. This result implies that secondary bile acids are caught and brought out by HMF, or rather its derivative "resistant protein," so as not to keep contact with colonic mucosae.