The in vitro effects of the bioflavonoid antioxidant silymarin on the expression and activity of superoxide dismutase (SOD) enzyme was studied in erythrocytes and lymphocytes from patients with chronic alcoholic liver disease. In vitro incubation with the agent in a concentration corresponding to the usual therapeutic dosage markedly increased the SOD expression of lymphocytes as measured by flow-cytofluorimetry following staining with monoclonal anti-Cu/Zn-SOD antibody and FITC-conjugated anti-mouse Ig, as well as erythrocyte and lymphocyte SOD activities. The data indirectly suggest that antioxidant activity might be one of the important factors in the hepatoprotective action of this bioflavonoid.