The modifying effect of dietary Peucedanum japonicum (PJ), which is a traditional herb in the Ryukyu Islands and is an anti-oxidant, on azoxymethane (AOM)-induced rat colon carcinogenesis was examined. Male F344 rats were divided into six groups: rats in groups 1-4 were given subcutaneous injection of AOM (20 mg/kg body weight) once a week for 2 weeks. Rats in groups 2, 3 and 4 were fed the diets containing 0.2 and 1% PJ and 0.025% chlorogenic acid, respectively. We observed modification of the preneoplastic lesions of both aberrant crypt foci (ACF) and beta-catenin accumulated crypts (BCAC) in colon carcinogenesis, microscopically and immunohistochemically. The numbers of ACF consisting of more than four aberrant crypts per rat in groups 2 (3.2+/-1.7) and 3 (3.0+/-3.2) were significantly lower than that of group 1 (10.8+/-4.9; P<0.05, respectively). The mean number of BCAC in both groups 2 (0.88+/-0.48/cm2/rat) and 3 (0.81+/-0.34/cm2/rat) was significantly lower than that in group 1 (2.13+/-0.54/cm2/rat; P < 0.0001, respectively). In addition, proliferating cell nuclear antigen labeling indices in group 2 (10.98+/-2.03) and group 3 (9.85+/-2.62) were significantly lower than that in group 1 (14.87+/-3.93; P < 0.001 and P < 0.0001, respectively). These findings indicate that PJ inhibits both ACF formation and accumulation of beta-catenin, and that PJ also reduces the cell proliferation activity, suggesting that PJ may have chemopreventive potential for colon carcinogenesis.