It is now well documented that bacterial mutagens form in proteinaceous foods during cooking at moderate temperatures. Three heterocyclic amine mutagens have been identified by Sugimura and coworkers in fish and beef cooked under moderate heating conditions (T. Sugimura and S. Sato, Cancer Res. (Suppl.), 43: 2415s-2421s, 1983). The distribution of these known mutagens in commercial bacteriological-medium grade and food-grade beef extract, and in fried ground beef, is discussed. Of the known mutagens, we have been able to confirm only that 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline is present in fried ground beef. Deficiencies in currently used mutagen extraction procedures are addressed. It is likely that there are many mutagens in fried ground beef that are yet to be identified. Fried ground beef (and raw beef) also contains an activity which modulates bacterial mutagenesis apparently by interacting with rat liver microsomes which are added to metabolically activate promutagens. The modulator activity has been partially purified and either inhibits, enhances, or has no effect on promutagen activation depending on the promutagen under study and the pretreatment of the rat from which the microsomal fraction was obtained.