The interaction between dietary fat, type and amount, and the dietary antioxidant butylated hydroxytoluene (BHT) on tumor induction and tumor growth by 7,12-dimethyl-benz[a]anthracene was studied. The study was to determine whether ingestion of polyunsaturated fat decreased or antagonized the inhibitory action of the antioxidant in comparison to diets that either contained equivalent amounts of a saturated fat or were very low in fat. The rationale was based on the known increased requirement for antioxidant caused by elevated levels of dietary unsaturated fat. Because the feeding of excess dietary polyunsaturated fat without an adequate elevation of dietary antioxidant causes various types of tissue injury in different species, it is reasonable to consider that the promotional effect of polyunsaturated fat in tumor development may be related to this phenomenon. We have confirmed that Sprague-Dawley female rats fed the high fat diets not only had a higher tumor incidence but also had a considerably higher tumor growth rate than did rats fed the low fat diet. Tumors also grew most rapidly in animals fed the diet high in polyunsaturated fat. When the antioxidant BHT was included in the diets, all 3 dietary groups showed a decrease in tumor incidence and rate of tumor growth. Whereas BHT completely eliminated the increased tumor incidence caused by high levels of saturated fat, it was much less effective in reducing tumor incidence in animals fed the same level of polyunsaturated fat. High levels of polyunsaturated fat exerted a tumor-enhancing effect, part of which was related to unsaturation that could not be eliminated by the antioxidant supplement.