Recent reports [t-4] of the detection by radioimmunoassay of the polypeptide hormones gastrin and somatostatin in gastric and duodenal juice suggested the possibility of the release of pancreatic hormones into the lumen of the pancreatic duct. Indeed, preliminary studies have demonstrated the presence of immunoreactive pancreatic polypeptide in human pancreatic juice [5] and immunoreactive insulin and glucagon in canine pancreatic juice [8]. Pancreatic juice, however, contains high concentrations of proteolytic enzymes and their zymogens, that can damage the labeled antigen or antibody used in radioimmunoassays[7] and result in artifactual hormone immunoreactivity. This study was, therefore, designed to isolate and characterize the immunoreactive components of the islet hormones, insulin, glucagon, somatostatin and pancreatic polypeptide in canine pancreatic juice.