Microfibril wheat bran (MFW), a processed dietary fiber prepared by milling of coarse wheat bran (WB), is softer and has a more pleasant taste than WB. In this study, we examined the inhibitory effect of MFW on azoxymethane (AOM)-induced colon carcinogenesis in female CF1 mice and compared its effect with that of WB and cellulose (CL). The mice were fed a modified AIN 76 A diet supplemented with either MFW, WB, or CL at a final concentration of 20% (w/w). Six weekly s.c. injections of AOM (10 mg/kg body weight) were administered per mouse commencing 1 week after the start of the feeding period. Control mice were injected with saline only. Thirty-three weeks after the initial injection, the mice were sacrificed, examined for tumors, and the cecal contents were analyzed to determine the moisture content and the concentrations of short-chain fatty acids (SCFA). The average number of total tumors per mouse in the MFW (2.9 +/- 0.6, P = 0.017) and WB (5.3 +/- 1.3, P = 0.373) diet groups was lower than that of the CL diet group (7.5 +/- 1.3), though there was no significant difference in tumor incidence (94.7%, 90.0% and 94.7%, respectively) between the groups. More than 90% of the tumors in each group were adenocarcinomas. The incidence of adenoma and that of carcinoma in situ in the MFW diet group (5.3% and 0%, respectively) were also lower than those in the CL diet group (26.3 and 26.3%, respectively; P = 0.180 and P = 0.046, respectively). Analysis of the cecal contents revealed a significantly higher moisture content and significantly higher concentrations of SCFA, butyrate in particular, in the MFW and WB diet groups. The results of this study indicate that the source and texture of dietary fiber can influence tumor development in CF1 mice, and more specifically that MFW is a promising and useful dietary supplement with properties serving to protect against the development of colon cancer.