The scavenging capacity of reactive oxygen species, such as hydroxyl radicals, is reported not to decreasein boiled garlic (an odorless garlic preparation). We therefore examined the modifying effect of boiled garlicpowder (BGP) on 1,2-dimethylhydrazine-induced mucin-depleted foci (MDF) and aberrant crypt foci (ACF),preneoplastic lesions, in the rat colorectum. Male F344 rats (5 weeks old) were fed a basal diet, or experimentaldiets containing 5% or 1% BGP for 5 weeks. One week later, all rats were injected s.c. with DMH (40 mg/kg, onceweekly for 2 weeks). At 10 weeks of age, all the rats were sacrificed, and the colorectum was evaluated for MDFand ACF. In rats given DMH and the 5% or 1% BGP diets (Groups 2 and 3), the numbers of MDF decreasedsignificantly in a dose-dependent manner, compared with the DMH and basal diet value (Group 1) (p<0.01). Thenumbers of ACF in Group 2, but not Group 3, showed a non-significant tendency to decrease. Next, the effectsof BGP on the formation of DMH-induced O6-methylguanine (O6-MeG) DNA adducts in rats were studied. MaleF344 rats (5 weeks old) were fed the basal diet, or 10% BGP diet for 5 weeks. All rats were injected i.p. once with40 mg/kg DMH at the end of week 5. The animals were sacrificed 6 hours after DMH injection to analyze theO6-MeG DNA adducts in the colorectal mucosa. Dietary administration of BGP significantly inhibited the O6-MeG DNA adduct levels in the colorectal mucosa, compared with the controls (p<0.01). These results suggestedthat BGP may exert chemopreventive effects against colon carcinogenesis at least in the initiation stage.