Epirubicin HCl is a new anthracycline analog and derivative of doxorubicin. Doxorubicin is a potent anticancer agent, the use of which is limited by its cumulative dose-dependent cardiotoxicity. Epirubicin HCl has more favorable therapeutic index than doxorubicin and possesses less hematologic and cardiac toxicity at comparable doses. Hepatoma G2 cells are a valuable model to study hepatocellular carcinoma and the liver, where drugs are metabolized. The goal of our study was to evaluate the-cytotoxic effect of epirubicin HCl on viability of Hep G2 cells measured using the MTT cytotoxicity test. Epirubicin HCl produced a concentration- and time-dependent cytotoxicity to Hep G2 cells. The mechanism of cytotoxicity of epirubicin HCl (IC50 value of 1.6 mug/ml within 24 h) appeared to involve a production of free radical species since activities of free radical scavenging enzymes (SOD, catalase, Se-dependent GPx) were increased. Addition of SOD prevented cytotoxicity of epirubicin HCl, and also counteracted the apoptosis. DNA fragmentation was determined to evaluate apoptosis. Western blot analysis indicated a decrease in GST-pi expression and increased activity of NADPH-dependent cytochrome P450 reductase which is a major enzyme in the conversion of epirubicin HCl to a free radical. It is proposed that production of reactive oxygen species increased by the treatment with epirubicin HCl can cause lipid peroxidation, which subsequently promotes apoptosis and reduces cell viability. Superoxide dismutase, catalase and glutathione peroxidase must be considered as a part of the intracellular antioxidant defense mechanism of Hep G2 cells against single electron reducing quinone-containing anticancer antibiotics.