The synthesis of phytochelatins (PCs) in a marine alga, Dunalliela tertiolecta, is strongly induced by Zn. Pretreatment of the cells with Zn enhances the tolerance toward toxic heavy metals such as Cd, Hg. Cu, Pb, and arsenate. Moreover, the pretreatment also increases the tolerance toward oxidative stress caused by hydrogen peroxide or paraquat. In vitro analysis shows that PC is a stronger scavenger of hydrogen peroxide and superoxide radical than glutathione. These results Suggest that PCs inducibly synthesized by Zn treatment could play a role not only in detoxification of heavy metals but also in mitigation of oxidative stress. (C) 2002 Elsevier Science (USA). All rights reserved.