As variation in both type of fibre and its physical properties can influence physiological effects, the effects of different dietary levels (1, 4, 8%, w/w) of unprocessed wheat bran (WB) were compared with those of two of its processed commercial formulations used in breakfast cereals, on the formation of aberrant crypt foci (ACF) and colon tumours in Fischer 344 rats following azoxymethane (AOM) administration. All diets were high in fat (20 g/100 g) and low in calcium (0.2%, w/w). The rats were fed the experimental diets for 2 wk before receiving two sc injections of AOM (15 mg/kg body weight/wk). 8 wk following the first injection of AOM, five rats per group were killed and the formation of ACF was measured. 23 wk following the first injection of AOM, 12 rats per group were killed and the colon tumour incidence in different dietary groups was measured. The results showed that increasing the dietary concentration of fibre from 1 to 8% (w/w), using all the wheat bran formulations, significantly reduced the number of ACF per rat. None of the diets showed any significant effect on the normal growth of rats. No statistically significant differences were observed between the protective properties of WB and the two commercial formulations under investigation in terms of the reduction of the number of ACF, or in terms of the reduction of the colon adenocarcinoma incidence. The results suggest that wheat bran and its two commercial formulations can offer protection against colon cancer even when they are consumed with a high-fat/low-calcium diet. The addition of any of these formulations of wheat bran fibre is likely to be equally effective in the prevention of colon cancer in human populations that habitually consume high-fat/low-fibre Western-style diets.