Myeloperoxidase catalyzes the formation of hypochlorous acid (HOCl) via reaction of $H_2O_2$ with $Cl^-$ions. Although HOCl plays a major role in the human immune system by killing bacteria and other invading pathogens, excessive generation of this oxidant causes damage to tissues. Exposure of HeLa cells to HOCl decreased viability, inactivated antioxidant enzymes, damaged mitochondria, and modulated cellular redox status. HOCl also induced significant increases in cellular oxidative damage reflected by lipid peroxidation, protein oxidation, and DNA damage. HOCl-mediated oxidative damage to HeLa cells may perturb the cellular antioxidant defense mechanisms and subsequently lead to a pro-oxidant state.