Patients with hepatobiliary pathology showed an augmentation of peroxide oxidation of lipids (LP), its degree dependent upon the severity of the clinical course of the medical condition in question, being the highest in those patients with viral hepatitis, carcinoma of the head of pancreas, and portal cirrhosis of the liver. A parallel investigation into the blood serum peroxidase activity, the main indicator of the antioxidant activity, in the above patients, showed a noticeable decrease in PA activity. LP and blood serum PA measurements allow some judgement about the gravity of the course of hepatobiliary pathology and effectiveness of the therapy adopted.