Because of the concern about environmental chemicals with oestrogenic and anti-oestrogenic effects, there is a need to construct biosensors for classifying such chemicals according to their effect on oestrogen receptor conformation. The conformation of the ligand-binding domains (LBD) of oestrogen receptor-alpha and -beta determine their transcription regulation activity. Some ligands, i.e., the natural oestrogen oestradiol, induce an active conformation allowing interaction with co-activators. In contrast, antagonists like ICI 182, 780, because of their bulky side chains, do not allow an alpha-helix 12 positioning compatible with co-activator binding. Another type of oestrogen receptor-ligand interactions, termed "passive antagonism", was first defined by X-ray crystal structure analysis of receptors in complex with the side chain-less 5,11-cis-diethyl-5,6,11,12-tetrahydrochrysene-2,8-diol (THC). We have now used the ability of peptides selected from phage-displayed peptide libraries to bind conformation specifically to oestrogen receptor-alpha and -beta LBDs to analyse conformations induced by THC and a group of chlorinated biphenyls and their aryl-hydroxylated metabolites, suspected of being environmental chemical disruptors. In oestrogen receptor-beta, THC defined a "passive antagonist" peptide recognition pattern, which was also induced by several antagonistic hydroxylated biphenyls, while a clearly different peptide recognition pattern was induced by their chlorinated agonistic counterparts. In oestrogen receptor-alpha, THC induced a conformation similar to that induced by oestriol and other oestrogen receptor-alpha agonists, which, as evaluated by site-directed mutagenesis, have a functionally important interaction with oestrogen receptor-alpha residue His524. We conclude that the peptide recognition pattern can be used to classify suspected environmental endocrine disruptors according the oestrogen receptor-alpha and -beta conformations they induce.