Commercially available ground rosemary powder was examined for its ability to modify the in vivo binding of 7,12-dimethylbenz(a)anthracene (DMBA) metabolites to mammary cell DNA in 55-d-old rats fed diets containing varying quantities and types of lipids. Supplementing a casein-based diet containing 20% corn oil with 1 % rosemary for 2 wk reduced by 76% the occurrence of DMBA-induced DNA adducts occurring 24 h after treatment with 50 mg DMBA/kg body weight. A comparable reduction in DNA adducts (66%) occurred when 0.5% rosemary was added to a diet containing 20% corn oil, and the quantity of DMBA given was reduced to 25 mg/kg body weight. The reduction in the occurrence of adducts occurring 24 h after DMBA treatment caused by 0.5% dietary rosemary was greater (P < 0.05) when added to a diet containing 20% corn oil than when added to a diet containing 5% corn oil and 15% coconut oil. Rosemary, 1% but not 0.5%, reduced DMBA-induced DNA adducts when the diet contained 5% corn oil. These studies demonstrate that rosemary is effective in reducing the binding of DMBA metabolites to rat mammary cell DNA. Furthermore, the present studies demonstrate that the benefits of rosemary are dependent on the source and concentration of dietary lipids.