7B2 is a novel neuroendocrine polypeptide which belongs to an entirely new superfamily of proteins. In extension of previous reports on 7B2, these studies concern its expression in endocrine pancreatic tissue. They have been performed using specific antibodies prepared against two distinct synthetic fragments of 7B2 comprising amino acids 23-39 and 117-128 of the native human molecule isolated from pituitary gland. Pancreatic insulin-secreting tumors produced in transgenic mice contain high amounts of 21,500- to 22,000-dalton forms of 7B2. Using light microscopy (immunocytochemical colocalization with different pancreatic hormones), immunoreactivity to 7B2 (IR-7B2) was consistently found within cells producing insulin and glucagon and less consistently within pancreatic polypeptide-containing cells. As in previous reports concerning the brain, adenohypophysis, and thyroid gland, IR-7B2 could be detected by electron microscopy within secretory granules of alpha- and beta-like cells in islets. Furthermore, the IR-7B2 level was higher in extracts of insulin-producing tumors of the transgenic mice that contained the hybrid insulin II gene. In addition, IR-7B2 could be detected immunocytochemically in three of seven tumors produced in the rat by streptozotocin-nicotinamide treatment.