Bisphenol A (BPA) and its major metabolite BPA glucuronide (BPA-G) were assessed for their ability to compete with [³H]17b -estradiol (E2) for binding to estrogen receptor (ER) a and b , and for inducing and antagonizing ERa - and b -mediated reporter gene activity in vitro.BPA competed with [³H]E2 for binding to mouse uterine cytosol ER, a bacterially expressed glutathione-S-transferase (GST)-ER fusion protein consisting of the human ERa D, E and F domains linked to GST (GST-hERadef) and a commercially available full length recombinant hERb .The IC₅₀ values for E2 were similar among receptor preparations (mouse uterine cytosol ER, GSThERa def and hERb were 3.0 ± 0.2 x 10^-9 M , 2.2 ± 1.4 x 10^-9 M and 5.5 ± 1.9 x 10^-9 M, respectively). In contrast, BPA exhibited greater affinity for hERb (3.3 ± 1.7 x 10^-7 M) than for GST-hERadef (2.7 ± 1.4 x 10^-6 M) and for mouse uterine cytosol ER (2.6 ± 1.1 x 10^-5 M).BPA-G did not competitively displace [³H]E2 from any of the ER preparations. In MCF-7 cells transiently transfected with Gal4-hERadef (human) or Gal4-mERbdef (mouse) chimeric receptors and a Gal4-regulated luciferase reporter gene, 17m5-G-Luc, BPA fully induced luciferase reporter gene activity with comparable EC₅₀ values (7.1 ± 2.9 x 10^-7 M and 3.9 ± 1.2 x 10^-7 M, respectively). No significant response was seen for BPA-G. Cotreatment studies showed that concentrations of (1-30 mM) BPA and BPA-G did not result in any significant changes in the level E2 (10 nM) induced luciferase activity in MCF-7 transiently transfected with either Gal4-hERa def or Gal4-mERb def.These results demonstrate that BPA can interact with ERa and b , and can induce ERa - and b -mediated gene expression in vitro. In contrast, BPA-G did not exhibit any in vitro estrogenic activity. This research was supported by the Bisphenol A Global Industry Group.