Olive oil contains several phenolic compounds possessing antioxidant activity. The aim of this study was to investigate the protective effects of olive oil phenolic extract (OOPE) and one of its constituents, gallic acid (GA) against H(2)O(2)-induced oxidative stress and apoptotic cell death in HeLa cells, a model for human epithelial cells.The cells were pretreated with nontoxic doses of OOPE or GA for 4, 24 and 48 h, and the intracellular reactive oxygen species (ROS) level was determined, before and after oxidative stress induction with H(2)O(2). As an indicator of apoptosis, caspase 9 activity was measured.All pretreatments reduced ROS generation. Four hour incubation with OOPE or GA completely inhibited ROS generation. Increases in caspase 9 activity by OOPE and GA pretreatment under harsh stress conditions were inhibited 92 and 67.8%, respectively.These results suggest that OOPE and GA act as powerful antioxidants against oxidative stress and exert anti-apoptotic effects.