Inbred female C3H/St mice exhibit the normal incidence of spontaneous mammary adenocarcinoma of 80--100% if they are maintained on a standard commercial laboratory diet containing 0.15 ppm of selenium with meat and dried skimmed milk as major sources of protein. The tumor incidence drops to 42% if animals of the same strain are kept on a diet containing 0.45 ppm of selenium, with fishmeal as the main source of protein. The tumor incidence declines further to 25, 19 and 10% if the animals in addition receive 0.1, 0.5, and 1.0 ppm of selenium in the drinking water. Selenium supplementation at these levels has no noticable adverse effects on weight-grains and survival of the mice. Selenium supplmented groups of animals also remained tumor-free for longer periods than the unsupplemented controls. The results of this study indicate that a diet rich in seafoods and cereals provides more selenium and may in turn lower the probability of cancer development. Reference is made to the average human diet in the U.S.A., which only contains 0.07--0.15 ppm of selenium due to the comparatively low consumption of cereals and seafoods. An equivalent mouse diet would not have any cancer-protecting effect in the C3H/St mice of our study. Australian workers have reported significantly lower tumor incidence in a different strain of C3H mice if it was kept in Australia rather than in the U.S.A. We have found that the Australian feed contained three times more selenium than that employed in the U.S.A. and propose that this difference in selenium content was primarily responsible for these previous observations.