The inhibitory effect of selenium on the genesis of spontaneous mammary tumors in C3H mice is statistically significant even at toxic levels of selenium (5 and 15 ppm of Se in form of selenite added to the supply water), no evidence for stimulation of tumor growth by selenium has been obtained. Arsenite lowers the tumor incidence at higher dosage (80 ppm of As in supply water) as well, but animals developing tumors under these conditions demonstrate significantly enhanced tumor growth rates. The addition of subtoxic concentrations of zinc (200 ppm in form of ZnCl2) to supply water containing 5 ppm of Se abolishes the cancer-protecting effect of selenium. The latter result is of possible importance with respect to the human breast cancer mortality experience: The calculated dietary zinc intakes of average adults in 28 countries correlate with the female age-corrected mortalities from breast cancer directly, with P less than 0.005. The zinc concentrations in whole blood from donors in different parts of the U.S.A. are also directly correlated with the female breast cancer mortalities.