Concern that some chemicals in our environment may affect human health by disrupt-ing normal endocrine function has prompted a research on interactions of environmental contaminants with steroid hormone receptor. Toxaphene and chlordane are among the 12 persistent organic pollutants identified by the United Nations Environment Programme as requiring urgent attention. We compared the estrogenic activity of two organochlorine pesticides, toxaphene and chlordane, at estrogen receptor a (ER$alpha$) and estrogen receptor $eta$ (ER$eta$). Human hepatoma cells (HepG2) were transiently transfected with rat ER$alpha$ or ER$eta$ plus an estrogen-responsive complement C3-luciferase (C3-Luc) reporter gene. After transfection, cells were treated with various concentrations of toxaphene and chlordane to investigate agonism or antagonism of these chemicals. Both toxaphene and chlordane were potent agonists in HepG2 cells for ER$alpha$. In contrast, these chemicals had a minimal agonist activity with ER$eta$ and almost abolished 17$eta$-estradiol-induced ER$eta$-mediated activity. Therefore, toxaphene and chlordane behaved as an ER$alpha$ agonist and an ER$eta$ antagonist with estrogen-responsive reporter plasmid C3-Luc, and exposure to these organochlorine pesticides could have a crictical effect on normal endocrine function.