Serum γ-glutamyl transpeptidase (γ-GTP) activities in patients with various hepatobiliary diseases were studied to elucidate the clinical significance of the enzyme. In an early stage of acute hepatitis, an elevation of the enzyme activity was slight or moderate, and the alteration of the activity in the clinical course behaved in a parallel manner with that of serum GPT activity. Estimation of the γ-GTP activity in a convalescent stage of acute hepatitis was useful for predicting the prognosis of the disease. A marked elevation of serum γ-GTP activity was observed in icteric primary hepatoma, especially when the tumor was large and situated at the hilar portion or in the right lobe of the liver, and in secondary carcinoma, the activity higher than that in primary hepatoma was obtained, when the cancer metastasized to the porta hepatis or infiltrated into the liver along the bile ducts even though jaundice was not revealed. In biliary diseases, the level of serum γ-GTP had a close correlation with those of alkaline phosphatase and leucine aminopeptidase.