Human CYP1A1, CYP1A2, and CYP1B1 mRNAs were constitutively expressed in MCF-7 (human breast carcinoma) cells and were extensively (6-12-fold) induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In contrast, in HeLa (human cervical adenocarcinoma) cells, CYP1A1 and CYP1B1 were induced by TCDD by up to 2-3-fold but CYP1A2 was not detected even when the cells were treated with TCDD. In the present study, the involvement of histone deacetylation and DNA methylation in the cell-specific inducibility of the human CYP1 family was investigated. The treatment of MCF-7 cells with trichostatin A (TSA), an inhibitor of histone deacetylase, and 5-aza-2'-deoxycitidine (AzaC), an inhibitor of DNA methylase, increased the constitutive expression level of CYP1A1, CYP1A2, and CYP1B1 by 2-3-fold. However, these treatments did not affect the levels of induction by TCDD. In HeLa cells, TSA and AzaC treatment increased the constitutive expression levels of CYP1A1 and CYP1B1. The induction of CYP1A2 was enhanced to a detectable level by TSA and AzaC even when the cells were not exposed to TCDD. Interestingly, pretreatment with TSA and AzaC increased the levels of CYP1A1, CYP1A2, and CYP1B1 induced by TCDD in HeLa cells. Furthermore, it was observed that TSA and AzaC treatment increased the constitutive expression level of AhR by 2-fold only in HeLa cells. To compare the methylation status of the 5'-flanking region of the human CYP1A1 gene including five XREs and the promoter region in MCF-7 and HeLa cells, the bisulfite-modified genes were amplified and sequenced. Since there was no remarkable difference in the methylation status within a -1.4 kb region of the human CYP1A1 gene, the methylation status in the CpG sites that exist in other regions of the human CYP1A1 gene might be involved in the cell-specific induction.