IBEP-2, an established cell line recently derived from breast carcinoma, was characterized with regard to estrogen receptor (ER) expression, cell mitogenic response to estrogenic stimulation and sensitivity to antiestrogens. In addition, we examined ER modulation following binding of agonist and antagonists, and the ER-mediated induction of progesterone receptor (PgR). ER level in IBEP-2 cells, determined by enzyme-linked immunoassay (EIA), was slightly higher than that measured in MCF-7 cells (662 v.s. 595 fmol/mg protein). When tested on IBEP-2 and MCF-7, various agonists stimulated cell growth with EC50's reflecting different estrogenic potencies (E2 ≈ diethylstilbestrol > E1 > genistein). IBEP-2 appeared slightly more sensitive than MCF-7, especially to E2 (at least 4-fold difference between EC50 values). By contrast, IBEP-2 and MCF-7 were equally sensitive to the growth inhibitory effect of antiestrogens 4-hydroxy-tamoxifen (OH-Tam) and ICI 182,780. As revealed by immunoblotting and immunofluorescence using anti-ERα antibodies, ER expression in IBEP-2 cells was modulated by E2 and estrogen antagonists like it has been shown in other ER-positive cell lines, that is, E2 and ICI 182,780 caused ER downregulation, whereas OH-Tam induced ER accumulation. Ligand-induced downregulation of ER involved degradation in proteasomes, since it was suppressed by the proteasome inhibitor MG-132. Exposure of IBEP-2 cells to E2 resulted in a marked (at least 25-fold) induction of PgR, documented by EIA, immunoblotting and immunofluorescence. PgR induction due to E2 was not modified by MG-132. Interestingly, MG-132 alone produced an ER-independent increase of PgR expression. IBEP-2 might prove to be valuable to study ER-mediated induction of PgR.