The clinical import of the serum gamma-glutamyl transpeptidase (GGTP) level was evaluated in 162 prospectively studied patients. GGTP is helpful in determining the origin of alkaline phosphatase (AP); it clearly separates increased AP of bone and placental origin from that of liver origin. The GGTP level closely parallels the AP level in most instances, but it may be more sensitive in detecting liver disease in anicteric patients. The finding of significantly increased GGTP in patients with chronic aggressive hepatitis as compared to normal levels found in chronic persistent hepatitis may provide a prognostic clue in cases of unresolved hepatitis. The apparent specificity and sensitivity of the GGTP test, combined with ease of performance and low expense, make it a valuable addition to the evaluation of a patient with hepatic disease.