Because mutagens typified by 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) observed in cooked foods are widely consumed, detailed studies of their biochemical and biological properties including carcinogenicity are most important. IQ induces unscheduled DNA synthesis in liver cells, which when taken together with its powerful mutagenicity in the Salmonella typhimurium test system, predicts carcinogenicity. In female Sprague-Dawley rats, IQ did exhibit potent carcinogenicity for the mammary gland, the ear duct, and to a lesser extent, pancreas and bladder. Data from Japanese laboratories indicate carcinogenicity also to the intestinal tract. Thus, one of the mutagens formed during cooking is a versatile carcinogen that because of extensive human intake requires urgent exploration for specific human cancer risk.