The B II bioassay was developed as a rapid and reliable tool for detecting potential insect growth regulators acting as ecdysteroid receptor (ant)agonists. Based on an ecdysteroid-responsive cell line from Drosophila melanogaster , this microplate assay is ideally suited to the evaluation of environmental contaminants as potential endocrine disrupters. Data are presented for about 80 potential environmental contaminants, including industrial chemicals, pesticides, Pharmaceuticals, phytoestrogens, and vertebrate steroids, and are compared with data for known (ant)agonists. Apart from androst-4-ene-3,17-dione (a weak antagonist), vertebrate steroids were inactive at concentrations up to 10 −3 M. The vast majority of xenobiotics also showed no (ant)agonist activity. Among the industrial chemicals, antagonistic activity was observed for bisphenol A median effective concentration (EC50) of × 10 −4 M and diethylphthalate (EC50 of × 10 −3 M). Some organochlorine compounds also showed weak antagonistic activity, including o,p′ -dichlorodiphenyldichloroethylene (DDE), p,p′ -DDE, dieldrin, and lindane (EC50 of × 10 −5 M). For lindane, bisphenol A, and diethylphthalate, activity is not associated with impurities in the samples and, for lindane and bisphenol A at least, the compounds are able to compete with ecdysteroids for the ligand binding site on the receptor complex, albeit at concentrations very much higher than those found in the environment. The only pharmaceutical showing any detectable antagonist activity was 17α-ethynylestradiol. In the context of recent publications on potential endocrine disruption in marine and freshwater arthropods, these findings suggest that, for some compounds (., diethylstilbestrol), ecdysteroid receptor-mediated responses are unlikely to be involved in producing chronic effects. The B II assay has a potentially valuable role to play in distinguishing between endocrine-mediated, which normally occur at submicromolar concentrations, and pharmacological effects in insects and crustaceans.