Abstract: A method which assists in the early diagnosis of rejection in a liver transplant recipient comprises measuring an increase in plasma or serum alpha glutathione S-.[.tranferase.]..Iadd.transferase.Iaddend.(.alpha.-GST) from said recipient in the absence of or preceding any change in plasma or serum transaminase. .alpha.-GST is most suitably measured by enzymeimmunoassay, using a solid phase antibody which is monospecific for .alpha.-GST. The monospecific antibody cross-reacts with the .alpha.-GST dimers B.sub.1 B.sub.1, B.sub.1 B.sub.2 and B.sub.2 B.sub.2.