y-Glutamyltransferase (GGT: E.C. 2.3.2.2.) is a glycoprotein enzyme which is involved in glutath- ione metabolism and amino acid transport through the plasma membrane. It is distributed widely in several organs including liver and kidney. Several isozymes of GGT have been reported and some of the isozymes may be associated with hepatocarcinogenesis. We have produced six monoclnal antibodies (mAbs) against GGT purified from the liver of 2-acetamidofluorene (AAF) treated rats. All of the six mAbs were obtained by immunizing mice with liver GGT. Six hybridomas which produced anti-GGT Abs were extensively subcloned and injected into the peritoneal cavity of BALB/c mice to obtain large quantities of Abs. These mAbs were purified from ascites by ammonium sulfate precipitation and protein A sepharose CL-4B column chromatography. Using these mAbs we proformed enzyme-linked immunosorbent assay (ELISA), radioimmvnoassay (RIA), immunohistochemistry (IHC), and autoradiography (ARG) to study the distribution of GGT isozyme in tissue. The results indicate that GGT-mAb 1 is specific for the AAF treated liver GGT, GGT-mAb 5 for the normal liver GGT, and GGT-mAb 6 for the normal kindey GGT. These mAbs may be used to evaluate the distribution of GGT isozymes in different tissues.