SummaryThe use of the determination of the gamma-glutamyl-transpeptidase (GT), the 5’nucleotidase and of the isoenzymes of the alkaline phosphatase for the differential diagnoses of liver disease is discussed.It is concluded that the GT is a useful test to diagnose diseases of liver, common bile duct or pancreas, but without distinction between these illnessess. An affection of these organs is not very probable if the GT is normal, it is very probable if the level of activity exceeds 500 units. In cancer patients without jaundice, a normal level of this enzyme suggests the absence of metastases of the liver. The enzymatic activity is normal in case of bone affection but can be elevated in case of some neurological diseases and 3 to 4 days after the beginning of a myocardial infarction.An abnormally elevated 5’nucleotidase accompanied with an elevated alkaline phosphatase suggests strongly the presence of an hepatobiliary damage, except when the alkaline phosphatase is very elevated, which may cause an“apparent”increase of the 5’nucleotidase and except when the elevation is due to a bone damage. The rise in case of bone damage seems however not to take place by more than 10 % of the cases. This enzyme is also useful for the diagnoses of active chronic liver damages.The determination of the isoenzymes of the alkaline phosphatase seems especially useful in case of an increase of the phosphatase alkaline whilst the 5’nucleotidase and the liver tests are normal. It often gives the possibility to determine whether the increase of the alkaline phosphatase in that case can be ascribed to a bone or to a liver damage. The technical details are important. The electrophoresis on acrylamide and the inactivation by heat seem to be adequate methods.To the autors it appears that the determination of the GT and of the 5’nucleotidase can find place in the classical liver tests, from one could eliminate other tests such as the Hanger. As far as the determination of the isoenzymes of the alkaline phosphatase is concerned, their application could be reserved for selected cases.