Cytochrome CYP1A1 gene expression, induced by polycyclic aromatic hydrocarbons and dioxins, eg. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is regulated mainly at the level of transcription. Inducible activation of the CYP1A1 promotor is mediated by a ligand-dependent transcription factor dimer complex including the aryl hydrocarbon receptor (AHR) and the AHR nuclear translocator (ARNT) proteins. Additional factors seem to be involved in tissue- and cell-specific modification of the induction process. In the present study HepG2 and MCF-7 cell lines were used to examine a possible cell-specific autoregulation of CYP1A1 promotor function. Chimeric CYP1A1-CAT reporter constructs and a human CYP1A1 cDNA expression plasmid were used in transient co-expression experiments. In HepG2 cells co-expression of increasing amounts of CYP1A1 cDNA significantly down-regulated constitutive as well as the TCDD-induced CYP1A1 promotor driven CAT activity. In contrast, co-transfection of MCF-7 cells with a 3-fold molar excess of CYP1A1 cDNA relative to the CYP1A1-CAT reporter construct caused an approximately 2-fold increase in the TCDD-induced CAT activity, whereas no effect was observed on constitutive promotor activity. This autoregulatory mechanism(s) of the human CYP1A1 gene product was independent of specific 5' flanking promotor segments tested. RT-PCR analyses did not indicate any changes in mRNA level of AHR and ARNT in the co-transfection studies. Thus these studies show that the human CYP1A1 gene is exposed to cell-specific autoregulation, probably achieved via different functions of trans-acting factors.