This study examined the effect of viable bifidobacteria and non-digestible carbohydrates on the cecal pH, colonic neoplastic lesion (aberrant crypt) and proliferating cell nuclear antigen (PCNA) labeling index in carcinogen-treated mts. Animals received s.c. injection of dimethylhydrazine (DMH) (15 mg/kg body weight) twice 3 days apart. Three days after the second carcinogen administration, the treatments were begun. 1he treatments were basal diet (AIN-76) with skim milk (Basal/skim), or the following diets with daily gavage of $10^8$ bifidobacteria: basal (Basal/bifido), $2\%$ fructo-oligosaccharide (FOS/bifido), $2\%$ soybean oligosaccharide (SBO/bifido), $2\%$ wheat bran oligosaccharide (WBO/bifido) and $8.4\%$ wheat bran (WB/bifido). After 4 weeks of treatment, cecal pH was measured using a pH probe. The number of aberrant crypt (AC), aberrant crypt foci (ACF) and crypt multiplicity were enumerated and colonic PCNA labeling index was determined using immunohistochemistry. Cecal pH was significantly reduced in SBO/bifido and FOS/bifido groups compared to control group. However, there were no significant differences in either number of AC or rates of cell proliferation as shown by PCNA labeling index among the groups, although mts fed FOS/bifido reduced the numbers of ACF compared to Basal/skim group. The SBO/bifido group did not reduce the number of ACF or PCNA labeling index. Also, other oligosaccharides did not reduce the risk of colon cancer compared to control group. The concomitant reduction of cecal pH and number of ACF suggest that the combination of bifidobacteria and FOS may reduce the risk of colon cancer.