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Di-ortho PCB congeners 52, 138, 153 and 180, and the mono-ortho coplanar congener 118 have been detected as a complex mixture in human tissue in Korea. This study examined the antiestrogenic effects of samples exposed to single or combination treatment of the ortho-PCB congeners. In order to determined the combined toxicity, a sample mixture (M1, M2, M3, M4, and M5) was designed based on the ortho-PCB congeners found in Korean human tissue. With the exception of PCB 52, the ortho-PCB congeners (PCB 118, 138, 153, and 180) showed weak antiestrogenic activity. The antiestrogenic activity of di-ortho PCB congeners (PCB 138, 153, and 180) was induced by the depletion of endogenous E2 as well as through the ER-dependent pathway, whereas the antiestrogenic activity of mono-ortho PCB 118 was only induced through the depletion of endogenous E2. When the MCF7-BUS cells were treated with mixtures containing the no effective concentration (10-6 M) of the PCB congeners, M3 (PCB 118 + PCB 138 + PCB 180) and M4 (PCB 118 + PCB 138) had an antiestrogenic effect but the other mixtures (M1; PCB 52 + PCB 118 + PCB 138 + PCB 180, M2; PCB 118 + PCB 138 + PCB 153 + PCB 180, M5; PCB 118 + PCB 180) did not. Although the mechanism for the interaction between the PCB congeners is not completely understood, it was presumed that exposure to a mixture of the PCB congeners might have synergistic effects on their antiestrogenicity through the ER-independent pathway.