alpha-Naphthoflavone (alpha NF) and 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) inhibited 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced CYP1A1 gene expression in MCF-7 human breast cancer cells and also decreased the accumulation of the nuclear [3H]TCDD-aryl hydrocarbon (Ah) receptor complex. Nuclear extracts from cells treated with 10(-6) M alpha NF and incubated with a dioxin responsive element (DRE, 26-mer) did not form a retarded band in a gel mobility shift assay. In contrast, incubation of nuclear extracts from cells treated with 10(-6) M MCDF and DRE gave a retarded band and this is consistent with the antiestrogenic and Ah receptor agonist activity of MCDF in human breast cancer cells. alpha NF was further investigated as an Ah receptor antagonist by determining the inhibition by alpha NF of TCDD-induced antiestrogenicity in MCF-7 cells. TCDD (10(-9) M) inhibited the 17 beta-estradiol-induced proliferation of MCF-7 cells and the secretion of the 52-kDa protein. In cotreatment studies, alpha NF (10(-8) to 10(-6) M) caused a concentration-dependent decrease in the antiestrogenic responses elicited by TCDD. In addition, alpha NF inhibited the TCDD-induced down-regulation of nuclear estrogen receptor levels in MCF-7 cells. alpha NF (10(-6) M) alone was inactive as an estrogen or antiestrogen and in cotreatment studies did not affect 17 beta-estradiol-induced responses in MCF-7 cells. Tamoxifen (10(-7) M), an antiestrogen which acts through the estrogen receptor, also inhibited 17 beta-estradiol-induced cell proliferation and alpha NF did not affect the tamoxifen-mediated antiproliferative response. Thus, alpha NF antagonized TCDD-induced CYP1A1 gene expression in MCF-7 cells and also acted as an anti-antiestrogen for TCDD-mediated antiestrogenicity in these cells. These results were consistent with the low levels of DRE binding observed with nuclear extracts from cells treated with 10(-9) M TCDD plus alpha NF (10(-8) to 10(-6) M) or 10(-6) M alpha NF alone. Thus, alpha NF appears to act as an Ah receptor antagonist in MCF-7 cells by decreasing the levels of transcriptionally active nuclear Ah receptor complexes.