Abstract: Lunasin is a novel peptide identified in soybean and other seeds. This study evaluated the anti-tumorigenic effects of lunasin on 7,12-dimethylbenz(a)anthracene (DMBA) and 3-methylcholanthrene-treated (MCA) fibroblast NIH/3T3 cells. Lunasin significantly inhibited cell proliferation and cancerous foci formation in these 2 chemical carcinogens-treated cells. An in vivo SENCAR mouse model induced with DMBA was used to study the mammary cancer preventive properties of dietary lunasin contained in soy protein. Tumor incidence was 67% and 50%, and the tumor generation was 1.88 ± 0.48 and 1.17 ± 0.17, respectively, for the mice fed control diet and experimental diet obtained after AIN-93G supplementation with lunasin-enriched soy protein concentrate (containing 0.23% lunasin). However, no effects were observed in mice fed AIN-93G supplemented with soy protein concentrate (containing 0.15% lunasin). The data provided illustrate the anticancer potential of lunasin both in vitro and in vivo and supports the recommendation of soy protein as a dietary component that may aid in the prevention of mammary cancer.