Twenty-four patients with normal or diseased hepatobiliary systems were subjected to a clinical trial in order to evaluate Tc-99m pyridoxal-phenylalanine as a biliary imaging agent. The investigation was combined with i.v. cholecysto-cholangiography in most cases, and the results were compared. It is concluded that qualitative information derived from conventional i.v. cholangiography is better in demonstrating the normal biliary tree or gallstones. The radiotracer techniques, however, are valuable in dynamic studies of Oddi's sphincter, in showing patency of biliary by-pass procedures, and in differentiating hepatocellular from obstructive jaundice.