To determine the cellular localization of glandular kallikrein in the human pancreas, immunohistochemical studies were performed with a monospecific antibody against the antigenically identical urinary kallikrein (urokallikrein). The localization of glandular pancreatic kallikrein to the beta cells of the islets was the same as that of insulin in normal human pancreas and in two islet-cell tumors. When beta cells were lacking in islet-cell tumors or in the pancreas of a patient with juvenile-onset diabetes, kallikrein antigen was not detectable. Anti-urokallikrein absorbed with purified urinary or pancreatic kallikrein no longer identified a pancreatic antigen, whereas absorption with insulin had no effect. The beta-cell localization of human pancreatic kallikrein, an endopeptidase that, in concert with carboxypeptidase B, converts bovine proinsulin to a polypeptide with the electrophoretic mobility of insulin, suggests that pancreatic kallikrein may be involved in the physiologic activation of proinsulin.