BACKGROUND: Chemical carcinogens such as benzo[a]pyrene (BaP) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) may contribute to the etiology of human diet-associated cancer. Individually, these are genotoxic, but the consequences of exposure to mixtures of these chemicals have not been systematically examined. OBJECTIVES: To determine the mutagenic response to mixtures of BaP and PhIP at concentrations relevant to human exposure (mM to sub-nM). METHODS: Human MCL-5 cells (metabolically competent) were exposed to BaP or PhIP individually or in mixtures. Mutagenicity was assessed at the thymidine kinase (TK) locus, CYP1A activity and message determined by Ethoxyresorufin-O-deethylase (EROD) activity and Q-PCR respectively, and cell cycle measured by flow cytometry. RESULTS: Mixtures gave modified dose-responses compared to the individual chemicals; a remarkable increased mutant frequency (MF) at low concentration combinations (not mutagenic individually), and decreased MF at higher concentration combinations, compared to the calculated predicted additive MF of the individual chemicals. EROD activity and CYP1A1 mRNA levels correlated with TK MF supporting involvement of the CYP1A family in mutation. Moreover, a cell cycle G2/M phase block was observed at high dose combinations, consistent with DNA damage sensing and repair. CONCLUSIONS: Mixtures of these genotoxic chemicals produced mutation responses that differed from expectations for additive effects of the individual chemicals. The increase in MF for some combinations of chemicals at low concentrations that were not genotoxic for the individual chemicals, and the non-monotonic dose response, may be important for understanding the mutagenic potential of food and the etiology of diet-associated cancers.