Two experiments were conducted to study the interaction of various diets with selenium in the genesis of mammary tumors. In the first experiment, virgin C3H mice were fed either an Oregon State University (O.S.U.) chow or this chow plus 0.5 or 2.0 ppm selenium in the drinking water, or Wayne Lab-Blox Chow with or without 2.0 ppm selenium in the water. Selenium significantly reduced the tumor incidence in mice fed the O.S.U. chow, but not in those fed the Wayne chow. In the second experiment, virgin female C3H mice were fed a purified casein-based diet with either lard, butter, corn oil, or rapeseed oil at a 10% level without or with selenium (2 ppm) in the drinking water. Selenium had no significant effect upon tumor incidence in this experiment. Thus, these results indicate that the type of diet has a marked influence on the beneficial effects of selenium in reducing mammary tumors in C3H mice.