We studied the effect of cooked food components on the promotion of microadenoma growth in the colons of mice and rats. CF1 mice and Fisher 344 rats were initiated with azoxymethane, with 152 mice receiving four weekly i.p. injections of 5 mg/kg, 59 rats receiving a single injection of 20 mg/kg, and 24 rats receiving 30 mg/kg. A week after the last injection, the animals were randomly assigned to one of eight diets with identical ingredients, but the three components, sucrose, casein, and beef tallow, either uncooked or cooked. Control animals were given diets with uncooked ingredients. Experimental animals were fed diets in which one, two, or three of the components were cooked in an oven at 180 degrees C until golden brown before they were added to the diet. After 100 days on the diets, the colons were fixed, stained with methylene blue, and scored for microadenomas. The mice and the rats fed cooked sucrose, or casein and beef tallow cooked together, had three to five times more large microadenomas than did the controls (P ranging from 0.02 to 0.0001). No significant increase was observed with the five other cooked diets. Two rats fed the casein and beef tallow cooked together had adenocarcinomas. Thus, a diet containing 20% of cooked sucrose, or 40% of casein and beef tallow cooked together, promotes the growth of colonic microadenomas in initiated mice and rats, and would appear to contain promoters for colon cancer.