OBJECTIVE This study aimed to evaluate the protein and lipid redox status in plasma erythrocytes and erythrocyte ghosts of alcoholics and of patients with non-alcoholic liver disease; we also investigated the relation to glutathione levels and the role of acetaldehyde and xanthine oxidase activity in plasma. METHODS Carbonyl and sulfhydryl proteins, glutathione and malondialdehyde levels and the activity of the circulating xanthine oxidase were determined in: active and abstinent alcoholics, patients with chronic viral hepatitis and healthy controls. RESULTS Active alcoholics showed a decrease of sulfhydryl protein and glutathione concentrations in plasma, erythrocytes and ghosts compared to the other groups. Also, an increase of the carbonyl protein and malondialdehyde levels and of the activity of circulating xanthine oxidase (9.2 +/- 1.8 nmol.min.ml, p < 0.001) were observed. Significant correlations between carbonyl protein and malondialdehyde concentrations in plasma (r = 0.775, p < 0.001), as well as between daily alcohol intake and carbonyl protein content in plasma (r = 0.879, p < 0.001) and erythrocytes (r = 0.605, p < 0.01) were observed. However, carbonyl protein levels did not correlate with the degree of liver injury. Incubation of plasma with acetaldehyde, but not with ethanol, significantly increased the carbonyl protein formation. Administration of N-Ethylmaleimide, a thiol depletor, or glutathione significantly increased or delayed, respectively, the carbonyl protein formation. CONCLUSIONS Proteins are oxidatively modified in plasma and erythrocytes of active alcoholics, whereas no such alterations are detectable in patients with non-alcoholic liver disease. Protein oxidation in alcoholics does not seem to result directly from ethanol; circulating xanthine oxidase, delivered from injured cells, may play a contributory role and glutathione appears to be directly involved in the protection of plasma proteins against acetaldehyde toxicity.