Abstract γ-Glutamyltransferase has been studied in bile and sera from patients with extrahepatic biliary obstruction. In both fluids γ-glutamyltransferase activity is found largely in the high molecular mass fraction present in the void volume following G200 gel chromatography and which remains at the origin after 7% polyacrylamide gel electrophoresis. When bile, sera and liver microsomes are treated with deoxycholate, a different form of γ-glutamyltransferase, with an approximate molecular mass of 157000 estimated by gel chromatography and of 125000 by gradient gel electrophoresis, is obtained. After treatment with papain, all three types of specimen gave rise to a form of γ-glutamyltransferase with a molecular mass of 115000 estimated by gel chromatography and of 98000 by gradient gel electrophoresis. The possible relationship between these various forms of γ-glutamyltransferase, and their relevance to the increases in activity in serum seen in extrahepatic biliary obstruction, are discussed.