The enzyme gamma-glutamyl transpeptidase is widely distributed throughout the body, notably kidney, seminal vesicles, pancreas, liver, spleen and brain. Being one of the enzymes of the gamma-glutamyl cycle, it is involved in aminoacid transport, catalysing a transpeptidation reaction between gamma-glutamyl peptides and most common amino acids. Methods of assay of the enzyme are based on its ability also to act on synthetic amides of glutamic acid; kinetic methods monitoring the release of p-nitroaniline from the substrate L-gamma-glutamyl p-nitroanilide are the most satisfactory. In diseases of the liver, the highest levels occur in association with cirrhosis, alcoholism, hepatic secondaries and cholestasis. As the enzyme is present in the endoplasmic reticulum of the hepatocyte, its activity is increased in situations leading to microsomal enzyme induction. Raised levels can also occur in pancreatitis, diabetes, myocardial infarction, congestive cardiac failure, chronic renal failure, cerebrovascular accidents, cerebral tumours and chronic obstructive pulmonary disease. Although the lack of specificity must be recognised, the estimation can be useful in the elucidation of some clearly defined problems arising during investigation of patients with suspected hepatic disease, especially where performed as part of a biochemical profile.