To gain a deeper insight into the potential interactions between individual aromatic hydrocarbons in a mixture, several benzoapyrene (BaP) and 7H-dibenzoc,gcarbazole (DBC) binary mixtures were studied. The biological activity of the binary mixtures was investigated in the HepG2 and WB-F344 liver cell lines and the Chinese hamster V79 cell line that stably expresses the human cytochrome P4501A1 (hCYP1A1). In the V79 cells, binary mixtures, in contrast to individual carcinogens, caused a significant decrease in the levels of micronuclei, DNA adducts and gene mutations, but not in cell survival. Similarly, a lower frequency of micronuclei and levels of DNA adducts were found in rat liver WB-F344 cells treated with a binary mixture, regardless of the exposure time. The observed antagonism between BaP and DBC may be due to an inhibition of Cyp1a1 expression because cells exposed to BaP:DBC showed a decrease in Cyp1a1 mRNA levels. In human liver HepG2 cells exposed to binary mixtures for 2h, a reduction in micronuclei frequency was also found. However, after a 24h treatment, synergism between BaP and DBC was determined based on DNA adduct formation. Accordingly, the up-regulation of CYP1A1 expression was detected in HepG2 cells exposed to BaP:DBC. Our results show significant differences in the response of human and rat cells to BaP:DBC mixtures and stress the need to use multiple experimental systems when evaluating the potential risk of environmental pollutants. Our data also indicate that an increased expression of CYP1A1 results in a synergistic effect of BaP and DBC in human cells. As humans are exposed to a plethora of noxious chemicals, our results have important implications for human carcinogenesis. •BaP:DBC mixtures were less genotoxic in V79MZh1A1 cells than BaP and DBC alone.•An antagonism between BaP and DBC was determined in rat liver WB-F344 cells.•The inhibition of CYP1a1 expression by BaP:DBC mixture underlies this antagonism.•A synergism between BaP and DBC was detected in human liver HepG2 cells.•The up-regulation of CYP1A1 expression was found in BaP:DBC-exposed HepG2 cells.