As the serum aspartate aminotransferase is unsatisfactory as an indicator of liver damage in cattle, the serum enzyme gamma glutamyltransferase was investigated. A normal range of 2–20 IU at 37° was established. Serum levels were raised following clinical facial eczema and in other cases of definite, or suspected, liver damage but not in situations involving muscle or kidney damage. Serum levels were raised for some months following clinical facial eczema. Mildly raised levels were found in apparently healthy cows in the 3 months following calving. An elevated serum gamma glutamyltransferase appears to be a sensitive specific indicator of liver damage, making it a useful diagnostic aid.