The 28 kDa peroxiredoxin from rat exhibited peroxidase activity only in the presence of dithiothreitol. Both organic and nonorganic peroxidases were found to be substrates for the 28-kDa peroxiredoxin activity. Analysis of the protective antioxidant activity of the 28-kDa peroxiredoxin revealed that it is accounted for by its peroxidase activity.