Enzymic activities of both γ-glutamyl transpeptidase and 5'-nucleotidase have been evaluated in patients in whose cases hepatobiliarγ-tree disease was suspected. γ-Glutamyl transpeptidase was a better indicator of diseases of the liver, bile duct, or pancreas than 5'-nucleotidase in nine of the ten conditions studied. 5'-Nucleotidase was found to be almost as useful as γ-glutamyl transpeptidase in indicating hepatobiliary disease in patients with granuloma of the liver, acute pancreatitis, congestive heart failure, rheumatoid arthritis, and carcinoma metastatic to the liver. γ-Glutamyl transpeptidase was most elevated in patients with primary carcinoma of the pancreas. Patients with carcinoma metastatic to bone or with neurologic conditions were found to have both enzymes elevated, but elevations occurred in a much lower proportion of patients than in those with hepatobiliary-tree disease. Liver scan findings correlate better with γ-glutamyl transpeptidase than with 5'-nucleotidase or alkaline phosphatase enzymic activity.