Effects of dietary calcium on mammary carcinogenesis in rats were investigated because of evidence that calcium counteracts the promotion of colon cancer by dietary fat and because experimental diets for rats normally contain higher amounts of calcium and vitamin D than do human diets. Our earlier experiments indicated that yields of tumors induced in young, Sprague-Dawley rats by 7,12-dimethylbenz(a)-anthracene (DMBA) were higher when dietary calcium, phosphate, and vitamin D were decreased. Results of an experiment in which dietary amounts of calcium, phosphate, and vitamin D were varied independently suggested that phosphate and vitamin D have interactive effects with calcium. Another experiment in which dietary vitamin D alone was varied provided evidence that higher amounts inhibited tumorigenesis in the presence of low amounts of calcium and phosphate but the results with a high-calcium and -phosphate diet were inconclusive. The findings suggest that low amounts of dietary calcium and vitamin D and high amounts of phosphate increase susceptibility to DMBA-induced mammary neoplasia.