To test the protective effect of herbal medicine against oxygen free radical-induced myocardiotoxicity, cytotoxicity of xanthine oxidase/hypoxanthine (XO/HX) was examined using MTT, TBARS, and beating rate assay in the presence of water extract of Sujeom-san(SJS) or single consituents of its prescription. Myocardial toxicity was evaluated in neonatal rat myocardiocytes in cultures. In the present paper, XO/HX resulted in a decrease in viability and beating rate and increases in lipid peroxidation in cultured myocardial cells. In the effect of SJS water extract, it showed effects from the cardiocytotoxicity induced by XO/HX treatment such as increases in beating rate and decreases in lipid peroxidation. In the effect of Rhizoma Corydalis (RC), Faeces Trogopterori (FT), Fructus Amomi Tsaoko (FAT) and Myrrha on the cardiocytotoxicity, they were significantly effective in blocking the XO/HX-induced cardiocytotoxicity by increase of beating rate in FAT and FT group as well as decrease of lipid peroxidation in FT and RC group. These results show that oxygen free radical elicits toxic effects in cultured myocardial cells derived from neonatal rat, and suggest that water extract of Sujeomsan, Rhizoma Corydalis, Faeces Trogopterori, Fructus Amomi Tsaoko or Myrrha is very effective in the prevention of xanthine oxidase/hypoxanthine- induced cardiotoxicity.