The comparative effects of different fibers on colonic luminal pH, crypt cell proliferation, and colon carcinogenesis were studied in 120 male Sprague-Dawley rats. The animals were divided into five equal groups and fed either a basal fiber free diet or the basal diet supplemented with 10% pectin, cellulose or guar, or 20% oat bran for up to 30 weeks. 1,2-Dimethylhydrazine was given at 20 mg/kg body weight as a weekly s.c. injection for 12 weeks. Food intake and weight gain were similar in all diet groups. At sacrifice, in vivo pH measurements showed that compared to fiber free rats, all fibers significantly acidified large bowel luminal contents (P less than 0.05). In the guar group 62.5% of rats developed colonic tumors compared to 33.4% of the fiber free rats (P less than 0.05). The yield of proximal colonic adenocarcinomas in the oat bran, pectin, and guar groups was increased by 4.5 to 5 times over the fiber free level (P less than 0.05-0.025). Pectin and guar provided the greatest stimulus to cell proliferation. A lower luminal pH was associated with a higher tumor yield and increased epithelial cell proliferation. Thus, acidification of colonic contents by high fiber diets failed to inhibit rat colon carcinogenesis, while the consumption of soluble fibers, such as oat bran, pectin, and guar, was associated with enhancement of proximal colon carcinogenesis.