In this report we describe the identification of a novel cell type in human and canine pancreas using tissue culture techniques. These cells, representing less than 1% of total islet cells, are of a small size (7-10 microm) and highly quiescent. They display a fairly immature morphology, which is characterized by a weakly developed protein synthesis machinery, a few mitochondria and a small number of neuroendocrine granules. These cells, which we have termed "small cells," are usually organized into small clusters, which can be identified within the islets of predominantly small size. They can also be collected as separate structures from preparations of freshly isolated islets. Immunohistochemically, small cells are positive for PDX-1, synaptophysin, insulin, glucagon, somatostatin, pancreatic polypeptide, alpha-fetaprotein and Bcl-2 and negative for cytokeratin 19 and nestin. Insulin secretion studies demonstrated that these cells secrete insulin in a glucose-responsive fashion, although do not respond to secretagogues such as IBMX and arginine as do mature beta cells. Although this study does not provide evidence of the proliferative and differentiation potential of small cells, their immature morphology, along with a small size and quiescence, let us hypothesize that these cells may serve as progenitors contributing to the islet growth.