Two groups of 20 male Sprague-Dawley rats each were given azoxymethane subcutaneously (8 mg/kg body wt) and fed a normal diet or one high in beef fat. Control groups were not given azoxymethane. The rats on the fat diet consumed less food and gained significantly more weight than the animals on the normal diet. Those given high fat and azoxymethane developed more intestinal tumors than did the dietary controls receiving the carcinogen. Furthermore, they had a greater number of larger tumors and more metastases than did the animals fed normally. No intestinal tumors were observed in control groups not receiving azoxymethane. The results show that the diet high in beef fat enhances the carcinogenic effect of azoxymethane in the rat.