This study examined the role of oxygen-derived free radicals for antitumor effects of intra-arterial injection with adriamycin (ADR). The effect of chemoembolization using ADR and degradable starch microspheres (DSM) was studied in rabbits with VX2 carcinoma of the hind leg. The tumor growth in rabbit treated with chemoembolization (ADR: 3.0 mg/kg + DSM 20mg/kg) was completely suppressed, but the therapeutic effect of chemoembolization combined with dimethylsulfoxide (DMSO) was reduced. On the other hand, the therapeutic effect of chemoembolization combined with superoxide dismutase and catalase was not reduced. These results indicate that the production of oxygen-derived free radicals, especially hydroxyl radical in the cancer cell, plays an important role in the antitumor effect of adriamycin.