Oxidative stress due to oxygen and various radical species is associated with the induction of DNA single- and double-strand breaks and is considered to be a first step in several human degenerative diseases, cancer and ageing. Naturally occurring antioxidants are being extensively analysed for their ability to protect DNA against such injury. We studied three naturally occuring compounds, Ascorbic Acid, Melatonin and Ellagic acid, for their ability to modulate DNA damage produced by two strong radical oxygen inducers (H2O2 and Bleomycin) in cultured CHO cells. The alkaline Comet assay was used to measure DNA damage and a cytofluorimetric analysis was performed to reveal the intracellular oxidative species. The data showed a marked reduction of H2O2- and Bleomycin-induced DNA damage exerted by Ellagic Acid. On the contrary Ascorbic acid and Melatonin appeared to induce a slight increase in DNA damage per se. In combined treatments, they caused a slight reduction of H2O2-induced damage, but they did not efficiently modulate the Bleomycin-induced one. The Dichlorofluorescein diacetate (DCFH-DA) cytofluorimetric test confirmed the strong scavenging action exerted by Ellagic Acid.