Various dietary components were evaluated as factors influencing garlic's ability to depress rat mammary cell DNA adducts resulting from 7,12-dimethylbenz(a)anthracene (DMBA) treatment. Diets with or without garlic powder (20 g/kg) were provided for 2 wk before DMBA treatment (25 mg/kg body weight). Rats fed diets containing 36 g casein/100 g diet had 31% fewer (P < 0.05) mammary cell DNA adducts than those fed 12 g/100 g. Garlic supplementation significantly (P < 0.05) reduced DNA adducts in rats fed either 12 or 36 g casein/100 g by 35 and 32% respectively. In the absence of dietary garlic, DNA adducts were 23% lower (P < 0.05) in rats provided a diet containing supplemental L-methionine at 0.9 g/100 g than at 0.3 g/100 g. However, adduct inhibition by garlic supplementation was greater in rats fed the lower (P < 0.05) amount of methionine (54 vs. 26% inhibition). Adduct levels in rats fed diets with 20 g corn oil/100 g were twice those occurring in rats fed 5 g/100 g (P < 0.05), regardless of adjustment for energy density. Garlic supplementation prevented the increase in DNA adducts caused by increasing dietary corn oil. Combining dietary supplements of garlic, selenite (0.5 mg/kg diet) and retinyl acetate (328 mg/kg diet) inhibited the occurrence of DNA adducts to a greater degree than when each was supplied individually. These studies demonstrate that while dietary garlic can reduce DNA adduct formation in mammary tissue caused by DMBA, this protection is influenced by several dietary components.