Exposure of Escherichia coli or mammalian cells to H2O2 results in cell death due to iron-mediated DNA damage. Since selenium compounds have been examined for their ability to act as antioxidants to neutralize radical species, and inorganic selenium compounds are used to supplement protein mixes, infant formula, and animal feed, determining the effect of these compounds on DNA damage under conditions of oxidative stress is crucial. In the presence of Fe(II) and H2O2, the effects of Na2SeO4, Na2SeO3, SeO2 (0.5-5000 microM), and Na2Se (0.5-200 microM) on DNA damage were quantified using gel electrophoresis. Both Na2SeO4 and Na2Se have no effect on DNA damage, whereas SeO2 inhibits DNA damage and Na2SeO3 shows antioxidant or pro-oxidant activity depending on H2O2 concentration. Similar electrophoresis experiments with [Fe(EDTA)](2-) (400 microM) and Na2SeO3 or SeO2 show that metal coordination by the selenium compound is required for antioxidant activity. In light of these results, Na2SeO4 may be safer than Na2SeO3 for nutritional supplements.