Gnotobiotes were produced by administrating Lactobacillus plantarum IAM 1041 in ICR strain male germfree mice which were fed by ordinary or high fat diet. Both groups were orally administered 0.3 mg/10 g of body weight (B.W.) of methylazoxymethanol (MAM) acetate. The oral administration of 0.3 mg/10 g/B.W. once a week for 11 consecutive weeks caused a total of 68 adenomatous polyps in the large intestine (an average of 11.4/mouse) of gnotobiotic high fat diet mice and a total of 32 adenomatous polyps (an average of 5.3/mouse) of the ordinary diet mice. There were no malignancies in either of the groups. Bile acids in the feces showed higher values in the high fat diet group than in the ordinary group. Bile acids are a factor which promotes the appearance of intestinal tumors. It was also assumed that the L. plantarum promoted the activation of beta-glucuronidase and alcohol dehydrogenase in the liver and intestine.