The in vitro and in vivo effects of the naturally occuring flavolignan hepatoprotective agent silibinin on the expression and activity of superoxide dismutase (SOD) enzyme were studied in lymphocytes from patients with chronic alcoholic liver disease. In vitro incubation with silibinin 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. In vivo treatment with the drug restored the originally low SOD activity of the patients' lymphocytes. These data indirectly suggest that antioxidant activity might be one of the important factors in the hepatoprotective action of silibinin.