Flavonoids are widely recognized as naturally occurring antioxidants. Naringin (NG) is one of the flavonoid components in citrus fruits such as grapefruit. Hydrogen peroxide (H₂O₂) causes cytotoxicity through oxidative stress and apoptosis. In this paper, we examined the effects of NG on H₂O₂-induced cytotoxicity and apoptosis in mouse leukemia P388 cells. Cytotoxicity was determined by mitochondrial activity (MTT assay). Apoptosis and DNA damage were analyzed by measuring chromatin condensation and Comet assay (alkaline single cell gel electrophoresis), respectively. H₂O₂-induced cytotoxicity was significantly attenuated by NG or the reduced form of glutathione (GSH), a typical intracellular antioxidant. NG suppressed chromatin condensation and DNA damage induced by H₂O₂. These results indicate that NG from natural products is a useful drug having antioxidant and anti-apoptopic properties.