This study examined the ability of environmentally relevant concentrations of 10 different polybrominated diphenyl ethers (PBDEs) to induce aryl hydrocarbon receptor (AhR)- and estrogen receptor (ER)-mediated gene expression in vitro. It also examined the ability of PBDEs to displace steroid hormones from serum proteins in vitro. At concentrations ranging up to 880 ng/ ml, none of the PBDEs significantly displaced tritiated 17beta-estradiol (E2) or testosterone from hormone-stripped carp serum. At concentrations ranging up to 500 ng/ml, 9 of 10 PBDEs tested failed to induce ER- or AhR-mediated gene expression in MVLN and H4IIE-luc cells, respectively. One congener, 3,3',4,4',5-pentabromodiphenylether (BDE 126), induced significant AhR-mediated gene expression at 500 ng/ml, but the magnitude of induction was only 13% of that caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Overall, the PBDEs tested were found to be at least 200,000 times less potent than TCDD and 50,000 times less potent than E2 for inducing AhR- and ER-mediated gene expression, respectively.