Reactive oxygen species (ROS), such as superoxide anion radicals (O.-2) and hydrogen peroxide (H2O2) are potentially harmful by-products of normal cellular metabolism that directly affect cellular functions. ROS is generated by all aerobic organisms and it seems to be indispensable for signal transduction pathways that regulate cell growth and reduction-oxidation (redox) status. However, overproduction of these highly reactive oxygen metabolites can initiate lethal chain reactions, which involve oxidation and damage to structures that are crucial for cellular integrity and survival. In fact, many antitumor agents, such as vinblastine, cisplatin, mitomycin C, doxorubicin, camptothecin, inostamycin, neocarzinostatin and many others exhibit antitumor activity via ROS-dependent activation of apoptotic cell death, suggesting potential use of ROS as an antitumor principle. Thus, a unique anticancer strategy named "oxidation therapy" has been developed by inducing cytotoxic oxystress for cancer treatment. This goal could be achieved mainly by two methods, namely, (i) inducing the generation of ROS directly to solid tumors and (ii) inhibiting the antioxidative enzyme (defense) system of tumor cells. Since 1950s, many strategies have been employed based on the first method, namely, administration of ROS per se (e.g. H2O2) or ROS generating enzyme to tumor bearing animals. However no successful and practical results were obtained probably because of the lack of tumor selective ROS delivery and hence resulting in subsequent induction of severe side effects. To overcome these obstacles, we developed polyethylene glycol (PEG) conjugated O.-2 or H2O2-generating enzymes, xanthine oxidase (XO) and D-amino acid oxidase (DAO) (PEG-DAO) respectively. More recently, a pegylated (PEG) zinc protoporphyrin (PEG-ZnPP) and a highly water soluble micellar formulation of ZnPP based on amphiphilic styrene maleic acid (SMA) copolymer, SMA-ZnPP, are prepared, which are potent inhibitors of heme oxygenase-1 (HO-1). HO-1 is a major antioxidative enzyme of tumors, that is different in mechanism of catalase or superoxide dismutase (SOD). Consequently, both PEG-enzymes and PEG-ZnPP exhibited superior in vivo pharmacokinetics than their parental molecules, particularly in tumor delivery by taking advantage of the EPR effect of macromolecular nature, and thus showed remarkable antitumor effects suggesting the potentials of this anticancer therapeutic for clinical application. Furthermore, it has been well known that many antioxidative enzymes such as catalase, SOD are down-regulated in most solid tumors in vivo. On the contrary, HO-1 is highly upregulated and it plays a very important role of antioxidation, because HO-1 generates biliverdin, which being converted to bilirubin exhibits a very potent antioxidative effect, and hence antiapoptosis in tumors. Thus this oxidation therapy, by inhibiting this HO-1 dependent antioxidant (bilirubin) formation by ZnPP, and by enhancing ROS generation, is expected to offer a powerful therapeutic modality for future anticancer therapy.