Heterocyclic aromatic amines (HAAs) are potent bacterial mutagens and potential human carcinogens formed in heat processed proteins. The Ames test (strain TA98) is a useful mutagenicity test system to screen food products for these compounds. HAAs require activation to their genotoxic forms, and in the Ames test, a rat liver S-9 preparation is normally used. In order to better understand the mechanisms of mutagen activation with respect to human metabolism, new bacterial strains containing human cytochrome P450s and other metabolic enzymes have recently been developed. We have investigated the capacity of one of these strains, DJ4309 [Josephy et al., Chem. Res. Toxicol. 11 (1998) 70-74] as a screening tool for mutagens in food products. DJ4309 expresses the human P450 1A2, human NADPH cytochrome reductase and the bacterial acetyl CoA:arylamine N-acetyltransferase. This strain is as sensitive as the Ames system to the mutagenic effects of the heterocyclic aromatic amines 2-amino-3-methylimidazo[4,5-f]quinoline, 2-amino-3,4-dimethylimidazo[4,5-f]quinoline and 2-amino-3,8-dimethylimidazo[4,5 f]quinoxaline, but less sensitive to 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. However, the mutagenicity of the arylamine 2-aminofluorene is considerably higher in DJ4309 than in the Ames test system. Meat extracts with a total HAA content ranging from less than 2 ng/g to 20 ng/g are efficiently detected by the Ames TA98 strain with rat liver S-9 activation. DJ4309 is less sensitive, with fewer revertants induced over the same dose range. Unknown compounds present in the meat extracts appear to inhibit the activity of the P450 1A2 enzyme in the DJ4309 strain. We have therefore demonstrated that although DJ4309 is a useful tool for mechanistic studies in chemical carcinogenesis, the screening of complex food matrices for HAAs by this bacterial strain must be conducted with caution. (C) 1999 Elsevier Science B.V. All rights reserved.