Magnesium-protoporphyrin IX (Mg-PPn), which is formed through chelation of protoporphyrin IX (PPn) with Mg ion by Mg chelatase, is the first intermediate for the (bacterio)chlorophyll biosynthetic pathway. Interestingly, Mg-PPn provides peroxidase activity (approximately $4{ imes}10^{-2}units/{mu}M$) detoxifying $H_2O_2$ in the presence of electron donor(s). The peroxidase activity was not detected unless PPn was chelated with Mg ion. Mg-PPn was found freely diffusible through the membrane of Escherichia coli and Vibrio vulnificus, protecting the cells from $H_2O_2$. Furthermore, unlike photosensitizers such as tetracycline and PPn, Mg-PPn did not show any phototoxicity, but rather it protected cell from ultraviolet (UV)-A-induced stress. Thus, the exogenous Mg-PPn could be used as an antioxidant and a UV block to protect cells from $H_2O_2$ stress and UV-induced damage.