The determination of gamma-glutamyltransferase (GGT) activity in the serum is commonly used as a screening test for alcoholism, since striking elevations of serum GGT activities can be observed in patients with a high alcohol intake over a prolonged period. Enhanced serum enzyme activities are also found in patients with various stages of alcoholic liver disease including alcoholic fatty liver, alcoholic hepatitis, alcoholic liver fibrosis and cirrhosis. Since enzyme alterations in the serum are commonly observed even during the early stage of alcoholic liver disease, such as alcoholic fatty liver, the determination of GGT activity in the serum is a useful test for early recognition of alcoholism. Moreover, the assessment of the adult and fetal form of GGT in the serum facilitates a clear dissociation between early stages of alcoholic liver diseases, such as alcoholic fatty liver, and late stages such as alcoholic liver cirrhosis. Recent studies have suggested that the activity enhancement in the serum is primarily due to hepatic enzyme induction, rather than to liver cell injury, and can be ascribed to the action of ethanol itself but not to dietary imbalance with respect to carbohydrates. The activity of GGT is increased after chronic alcohol consumption in plasma membrane and microsomal fractions of the hepatocyte. This suggests that GGT is primarily induced in the endoplasmic reticulum and subsequently transported to plasma membranes via Golgi apparatus and/or microtubuli. In the presence of ethanol, GGT of the plasma membrane may then be solubilized and released into the blood.