Abstract: The purpose of this study is to investigate the possible antimutagenic effects of whey proteins against genotoxic effects induced by aflatoxins in the mammalian cell system. The micronuclei, chromosomal abnormalities in both somatic and germ cells as well as DNA titrations in male rats were assayed. Whey proteins was added to the diet containing aflatoxins (2.5 mg AFs kg-1) and fed to mature male rats for 9 weeks. The results indicated that aflatoxin significantly (p<0.001) increased chromosomal abnormalities in somatic and germ cells, DNA titration and micronuclei of bone marrow cells. Addition of whey proteins to the aflatoxins contaminated diet resulted a significant decrease in chromosomal aberrations in somatic cells (62.8%, p<0.001), in germ cells (57.9%, p<0.01) and in micronuclei polychromatic erythrocytes (68.2%, p<0.001). It could be concluded that whey protein reduced the mutagenesis and clastogenesis effects of AFs. Although the mechanism of antimutagenicity is unknown, it seems plausible that the whey proteins acts as a blocking agent by chemical or physical interaction with the mutagens. Furthermore, this study revealed that the addition of whey proteins to aflatoxin contaminated diet diminished most of the genotoxic effects of the Aflatoxin.