γ-GT serum levels have been compared in various liver diseases with AP and LAP, cholestatic markers ; with GOT and GPT, the increases of which are related to cellular injury. γ-GT levels have also been assayed in serum and in hepatic tissue of rats after poisoning with CCl4 and after bile duct ligation. The results showed in all cases a significant correlation between γ-GT and AP, and γ-GT and LAP. A significant increase of γ-GT was found in plasma and in hepatic tissue of rats after bile duct ligation; on the other hand normal levels were found in the plasma of rats after treatment with CCl4. γ-GT gelfiltration on Sephadex G-200 showed one peak only of activity in all cases considered, eluted with the first protein fraction. The enzymes seem to be rather homogeneous with a molecular weight of about 200,000.