y-Glutamyl Transpeptidase, y-GTP was discovered in 1950 by Hanes et al. And isolated pure form from ox kidneys in 1963 by Szewczuk and Baranowski. Y-GTP has been found in number of tissues: kidney, pancreas, mammary gland, liver, intestine, lung, spleen, thyroid gland, bone marrow and lens. In human tissue the greatest activity has been found in the proximal tubules and in the loops of Henle in the kidney, and in the epithelium of the intrahepatic bile ducts and pancreatic ducts. An increase in y-GTP activity in serum has been observed virtually only with disease of liver, bile ducts and the pancreas. The y-GTP activity appears to have particular significance in liver diseases due to alcohol and in the detection of liver metastasis. We estimated serum p-GTP levels in 52 healthy Korean(male: 26, female:26). The normal value of y-GTP in the Korean was 10.1+5.3 IU/L(mean+SD). The mean of y-GTP level in the male was 13.2+5.7 IU/L(4.1-26.2 IU/L), and the mean of y-GTP level in the female was 6.9+2.2 IU/L(3.4-13.7 IU/L). There was sex difference of y-GTP acti vities.