Abstract The activity of γ-glutamyltransferase was studied in guinea pig after bile duct ligation. In serum, an abrupt increase in activity up to 10–20 times the normal value was found 3 h after obstruction and the mean activity over the first 3 days following the operation was some 8 times the normal value. In liver, however, a small decline in activity could be demonstrated. The administration of cycloheximide did not influence the acute increase in serum activity. Bile duct ligation caused marked increases in serum bile acid levels which initially paralleld the serum γ-glutamyltransferase activity. It is suggested that the increased serum activity may arise from the solubilization by bile acids of liver membrane-bound enzyme.