Polychlorinated biphenyls (PCBs) are one of the most widespread, persistent man-made products in the ecosystem giving rise to serious environmental contamination and potential hazard to health. The PCBs, in common with other compounds such as the dioxins, have been shown to exert some biological actions mediated through the aryl hydrocarbon receptor. Evidence for interaction of PCBs with other nuclear receptors has been sparse. Here we present evidence that 3,4,3',4'-tetrachlorobiphenyl (TCB) (PCB77), a PCB with high toxicity and significant bioaccumulation, can act as an estrogen with actions mediated through the estrogen receptor. Evidence is presented from multiple assay systems including 1) ligand binding to estrogen receptor in a competitive binding assay, 2) ligand ability to induce estrogen receptor binding to DNA, 3) ligand regulation of gene expression from a transfected exogenous (ERE-tk-CAT) or an endogenous (pS2) estrogen-regulated gene, 4) ligand regulation of cell growth in estrogen-dependent human breast cancer cell lines MCF7 and ZR-75-1, and 5) ligand activity in the immature mouse uterine weight bioassay in vivo. These results demonstrate that TCB (PCB77) can be included in the increasing list of environmental pollutants that possess the ability to mimic estrogen action and be termed an environmental estrogen. Since the concentrations of TCB used here (10(-9) M; 292 ng/liter) are not incompatible with levels of PCB/TCB found in human tissues, these results may have physiological relevance. Use of multiple approaches to study estrogenic action demonstrates that one congener can act as both an agonist and antagonist of estrogen action and that the magnitude of these effects can alter according to the molecular environment.