Human breast cancer cell lines are widely used to study the antiestrogenic effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in vitro. Like other groups we found that 10 nM TCDD inhibits cell growth and induces cytochrome P450 1A1 (CYP1A1)-associated 7-ethoxyresorufin-O-deethylase (EROD) activity in MCF-7 cells expressing the estradiol receptor (ER). Neither cell growth nor EROD activity was affected in ER-negative MDA-MB 231 cells. Results of reverse transcription-polymerase chain reaction (RT-PCR) revealed a strong induction of CYP1A1 mRNA in MCF-7 but only a weak increase in MDA-MB 231 cells treated with 1, 10, or 100 nM TCDD. Transcripts of CYP1B1 were detected in both cell lines and mRNA content was enhanced 8- and 30-fold in MCF-7 and MDA-MB 231 cells treated with 1 nM TCDD, respectively. In gel mobility shift assay a stronger signal of DNA-binding aryl hydrocarbon receptor (AhR) was observed in MDA-MB 231 than in MCF-7 cells treated with 10 nM TCDD. These results were confirmed by RT-PCR analyses which showed an approximately 40-fold higher AhR mRNA content in untreated MDA-MB 231 than in MCF-7 cells. In contrast the mRNA of the AhR nuclear translocator was expressed in a similar range of magnitude. Treatment of the cells with TCDD did not change mRNA expression of both genes. Analysis of NADPH:quinone oxidoreductase (NMO-1) and plasminogen activator inhibitor-2 (PAI-2) mRNA expression revealed a dose-dependent induction of both genes in MDA-MB 231 cells after TCDD-treatment. From the results it was concluded that AhR-mediated transactivation is not impaired in ER-negative MDA-MB 231 cells. In addition, the results confirm reported data that expression of ER seems to be important for regulation of CYP1A1 induction after TCDD in human breast cancer cell lines but the present data show that ER does not appear to have a function in TCDD-induced mRNA expression of CYP1B1, NMO-1, and PAI-2 in MDA-MB 231 cells.