This study was conducted to assess the chemopreventive effects of chitosan in a rat colon carcinogenesis induced by azoxymethane (AOM). Ninety, 5-week-old, male F344 rats were divided into three groups. The animals in group 1 received subcutaneous injections of 15mg/kg AOM three times for two weeks, then were placed on powdered basal diet containing 2% chitosan for 37 weeks from weeks 3 to 40. The animals in group 2 were given AOM alone. The animals in group 3 were given 2% chitosan without prior carcinogen treatment. All animals were sacrificed at week 12 for quantitative analysis of aberrant crypt foci (ACF) and at week 40 fur analysis of tumor induction. Total numbers of ACF and AC per colon of group 1 were not significantly different from those of group 2. Tumor incidences and multiplicities of small intestine in the group 1 were significantly decreased compared with those of the group 2 (P<0.05). According to pathological diagnoses, adenocarcinoma incidence and multiplicity in the small and large intestine in the group 1 were significantly decreased compared with those of the group 2 (p<0.05). No toxic effects were observed in animals given chitosan in terms of body weights, and liver or kidney histology. These results indicate that chitosan may have a potential as chemopreventive agents of colon carcinogenesis during the postinitiation stage.