The impact of estrogen receptor (ER) was examined for expression and activity of cytochrome P4501B1 (CYP1B1) and cytochrome P4501A1 (CYP1A1) in two pairs of ER+/ER- human breast epithelial cell lines derived from single lineages, and representing earlier (T47D) or later (MDA-MB-231) stages of tumorigenesis. Acute loss of ER was evaluated using the anti-estrogen ICI 182,780 (ICI). In all lines, CYP1B1 was expressed constitutively and was induced by 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), whereas CYP1A1 was expressed only following induction. Expression of each CYP (with or without TCDD) was greater in T47D cells than MDA cells. The ER impacted expression of these genes in opposite directions. The ER- phenotype was associated with less TCDD-induced CYP1A1 expression, but greater basal and induced CYP1B1 expression. A 48 h treatment of ER+ cells with ICI did not revert the P450 expression pattern to that of ER- cells. Based on activities of recombinant enzyme and expression levels, differences in 7,2-dimethylbenz [a]anthracene (DMBA) metabolism between the cell lines were consistent with differences in CYP1A1 and CYP1B1 expression. In T47D lines, basal microsomal DMBA metabolism was primarily due to CYP1B1, based on regioselective metabolite distribution and inhibition by anti-CYP1B1 antibodies (>80%). Metabolism in TCDD-induced microsomes was mostly due to CYP1A1 and was inhibited by anti-CYP1A1 antibody (>50%). TCDD-induced MDA+ cells demonstrated CYP1A1 activity, whereas TCDD-induced MDA- cells displayed CYP1B1 activity. Aryl hydrocarbon receptor (AhR) levels, but not AhR nuclear translocator protein (ARNT) levels were highly dependent on cell type; AhR was high and ER-independent in MDA, and low and ER-linked in T47D. AhR levels were insensitive to ICI. ER does not directly modulate the expression of CYP1A1, CYP1B1 or AhR. Indeed, factors that have replaced ER in growth regulation during clonal selection predominate in this regulation. Characteristics unique to each cell line, including ER status, determine CYP1A1 and CYP1B1 expression.