The effects of dietary polyunsaturated fat in BALB/c mice were determined for several stages of mammary tumorigenesis: a) growth of intact and transplanted normal mammary tissue, b) growth of hyperplastic alveolar nodule (HAN) lines D1/ UCD and Z5C1 , c) incidence of tumors occurring in D1/ UCD , and d) growth rate of the tumors. Growth of transplanted normal mammary epithelium as well as intact glands was faster in mice fed a 10% corn oil (CO) diet, which contains linoleate, than in those fed 10% hydrogenated cottonseed oil ( HCTO ), a diet free of the polyunsaturated fatty acid. We concluded that transplantation itself does not alter the response of mammary tissue to dietary fat. The growth of both HAN lines was unchanged by the presence of CO in the diet. For ascertainment as to whether the difference in response was specific to the tissue, ducts and HAN line D1/ UCD were transplanted into left and right gland-free fat pads of the same mice. The duct tissue grew faster when the hosts were fed the 10% CO diet than in the hosts fed the 10% HCTO diet; however, no effect of dietary fat was noted with the HAN. Although indomethacin retarded the growth of normal tissue in the mice fed CO diet, this prostaglandin inhibitor had no effect on the growth of the HAN transplants. Both incidence and growth rate of tumors arising from transplants of HAN line D1/ UCD were greater in mice fed diets containing either 0.3, 1, or 10% CO than in those fed 10% HCTO . The smaller number of tumors arising in mice fed with 10% HCTO diet may be the result of the slower growth rate of the neoplasms in mice fed diets devoid of linoleate.