Serum gamma-glutamyl transpeptidase is one of the enzymes in diagnosis of liver diseases, since a new colorimetric method was devised by Orlowski, M. et al. Forty mU/ml of serum gamma-glutamyl transpeptidase activity is said to be the upper limit at clinical level. When this value is set up as a screening level in the periodic health examination, about 35% of the subjects including daily drinkers can be evaluated as abnormal. In the present study, the upper limits of serum gamma-glutamyl transpeptidase activity were 102 mU/ml for 147 normal subjects including daily drinkers and 49 mU/ml in 70 non-drinkers selected from the subjects. Therefore, we propose that the standards for screening the abnormal from the normal in the periodic health examination should be 50 mU/ml for non-drinkers and 100 mU/ml for drinkers.