Food irradiation is acknowledged as a safe process to improve food quality by reducing microbial contamination. Information on the toxicological potential of 2-alkylcyclobutanones (2-ACBs), radiolytic derivatives of triglycerides found exclusively in irradiated food, is scarce. Wistar rats received daily a solution of highly pure 2-tetradecylcyclobutanone (2-tDCB) or 2-(tetradec-5-enyl)-cyclobutanone (2-tDeCB) at a concentration of 0.005% in 1% ethanol as drinking fluid, while control animals received 1% ethanol. All animals received a single intraperitoneal injection of the chemical carcinogen azoxymethane (AOM) at Weeks 3 and 4. At 3 mo after AOM injection, no significant changes were observed in the total number of preneoplastic lesions in the colon of AOM controls and 2-ACB-treated animals. After 6 mo, the total number of tumors in the colon was threefold higher in the 2-ACB-treated animals than in the AOM controls. The colon of four of six AOM control rats exhibited only one small tumor ( &6 mm3). Multiple tumors were observed in four and three of six animals treated with 2-tDCB or 2-tDeCB, respectively. Medium (6 < S < 25 mm3) and larger (>25 mm3) tumors were detected only in 2-ACB-treated animals. This is the first demonstration that a compound found exclusively in irradiated dietary fats may promote colon carcinogenesis in animals treated with a chemical carcinogen.