Cytokeratins (CKs) serve as immunocytochemical markers of epithelial cells. We found that duct cells of the neonatal and adult rat pancreas express CKs 7, 19, and 20. Because pancreatic endocrine cells are thought to derive from duct cells, we examined their expression of CKs 7, 19, and 20 during the neonatal period and the proliferative activity of the different cell types in and around islets. Immunocytochemical analysis revealed that the islets of the neonatal pancreas, in contrast to those of adults, strongly expressed CKs 7, 19, and 20, particularly within a peripheral mantle zone that was continuous with the epithelium of adjacent ductules. This pattern was found during the first 2 weeks of life when significant islet growth occurred as determined by morphometry and bromodeoxyuridine (BrdU) labeling. Moreover, BrdU labeling kinetics indicated that the growth of neonatal islets occurred in the peripheral mantle region characterized by intense CK20 and CK19 immunoreactivity and not in the region composed of differentiated endocrine cells. These observations strongly suggest that proliferating ductal cells associated with islets represent a pool of islet beta- and non-beta-precursor cells.