We analyzed whether polychlorinated biphenyls (PCBs) interfere with the activity of 17 β-estradiol in the proliferation and apoptosis of the MCF-7 cell line. MCF-7 cells were cultured in Dulbecco's modified Eagle's medium (DMEM) without phenol red supplemented with 5% charcoal-treated fetal bovine serum (CD-FBS) for 3 days with 10 nM 17 β-estradiol or 0.1 µM, 0.5 µM and 1 µM of the tested PCB congeners (118, 138, 153 and 180), or both. Cell proliferation was determined by measuring 5-bromo-2'-deoxyuridine (BrdU) incorporation, and cell apoptosis was measured by caspase-9 activity. From the PCB congeners tested, PCB138 and 153 had the highest stimulatory effects on basal cell proliferation as well as the highest inhibitory actions on basal caspase-9 activity. The proliferative and anti-apoptotic actions of PCB138 and 153 were still observed in the presence of 17 β-estradiol, while the actions of PCB118 and 180 were reversed. In conclusion, the results of this study suggest the possibility that PCB138 and 153 contribute to the action of endogenous 17 β-estradiol on cell proliferation and apoptosis in the breast cancer cell line MCF-7.