Treatment of estrogen receptor (ER)-negative MDA-MB-468 human breast cancer cells with 10 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced formation of a nuclear aryl hydrocarbon (Ah) receptor complex as determined by ligand-binding and gel electrophoretic mobility shift assays. TCDD also induced CYP1A1-dependent activity in MDA-MB-468 cells, which represents the first ER-negative Ah receptor-positive human breast cancer cell line that has been identified. Treatment of this cell line with TCDD and related compounds also caused a 50% inhibition of cell growth, which resembled the growth inhibitory effects previously reported for epidermal growth factor (EGF). However, EGF expression is minimal in this cell line and is not induced by TCDD; moreover, EGF and TCDD induced a different pattern of oncogene expression and apoptosis in MDA-MB-468 cells. In contrast, TCDD caused a rapid and sustained induction of transforming growth factor alpha (TGF alpha) gene expression and secreted protein (nearly 2-fold); moreover, the growth-inhibitory effects of TCDD could be blocked by antibodies to the EGF receptor. In a separate experiment, it was shown that TGF alpha also inhibited growth of MDA-MB-468 cells. The results of this study indicate that the mechanism of growth inhibition of MDA-MB-468 cells by TCDD is due to induction of TGF alpha, which is a potent antimitogen in this cell breast cancer line.