Effects of selenium and of arsenic on the genesis of spontaneous mammary tumors in inbred C3H mice.

Exposure of female virgin C3H /St mice to two ppm of selenite in the drink­ ing water for 15 months lowered the incidence of spontaneous mammary tumors to 10 percent relative to the observed incidence of 82 percent in untreated controls. Arsenic in the drinking water in form of arsenite, at levels of 10 ppm, reduced the tumor incidence to 27 percent but caused a significant enhancement of the growth rate of spontaneous or transplanted mammary tumors. The present results provide a pertinent example for the cancer pro­ tecting effect of the essential trace element selenium. A comparison of the female breast cancer death rate in the Continental United States indicates that the breast-CA mortality is lower in areas with abundant or high dietary selenium supply than in regions deficient in this element.