Persistent polychlorinated biphenyl (PCB) metabolites were studied with respect to their interaction with the human glucocorticoid receptor (GR). 3-Methylsulphonyl-2,5,6,2',4',5'-hexachlorobiphenyl (3-MeSO2-CB149) was shown to compete with 3H-dexamethasone for binding to the GR, with an IC50 (concentration that inhibits 50%) of approximately 1 microM. Using GRAF cells expressing human GR, glucocorticoid responsive element, and a reporter enzyme, we demonstrated that 3-MeSO2-CB149 functionally acts as an antagonist at the GR (IC50 = 2.7 microM). In accordance with the receptor binding, the antagonism mainly appeared to be of a competitive nature. When studying the competitive binding of 24 methylsulfonyl PCBs (relative to dexamethasone) to GR from mouse liver cytosol, seven compounds had a higher affinity to GR than 3-MeSO2-CB149. Structure-activity relationship studies indicated that the presence of three chlorine atoms in the ortho-position and chlorine and methyl sulfone groups on either end of the molecule (4 and 4'-position) increased the affinity to GR. The relevance of this finding for human health is not known, but PCB methyl sulfones are ubiquitous pollutants present in mother's milk. The results stress the need for studying endocrine disruptors that affect hormonal systems other than sex and thyroidogenic hormones.