A simple and rapid method suitable for clinical laboratories, for the removal of the inhibitors in urine of γ-glutamyl transpeptidase activity, using an ion-retardation resin, is described. Gel chromatography of urine resulted in appreciable loss of the enzyme activity. Freezing at -5°C and thawing, did not affect the activity of the enzyme. Dimethyl sulfoxide and polyvinyl pyrrolidone up to a concentration of 20% did not have any effect on the transpeptidase activity. Urea, creatinine and NaCl at levels found in normal urine did not inhibit the transpeptidase. © 1976.