Apoptosis is known to be induced by direct oxidative damage due to oxygen-free radicals or hydrogen peroxide or by their generation in cells by the actions of injurious agents. Together with glutathione peroxidase and catalase, peroxiredoxin (Prx) enzymes play an important role in eliminating peroxides generated during metabolism. We investigated the role of Prx enzymes during cellular response to oxidative stress. Using Prx isoforms-specific antibodies, we investigated the presence of Prx isoforms by immunoblot analysis in cell lysates of the MCF-7 breast cancer cell line. Treatment of MCF-7 with hydrogen peroxide (H 2 O 2 ) resulted in the dose-dependent expressions of Prx I and II at the protein and mRNA levels. To investigate the physiologic relevance of the Prx I and II expressions induced by H 2 O 2 , we compared the survivals of MCF10A normal breast cell line and MCF-7 breast cancer cell line following exposure to H 2 O 2. The treatment of MCF10A with H 2 O 2 resulted in rapid cell death, whereas MCF-7 was resistant to H 2 O 2 . In addition, we found that Prx I and II transfection enabled MCF10A cells to resist H 2 O 2 -induced cell death. These findings suggest that Prx I and II have important functions as inhibitors of cell death during cellular response to oxidative stress.