Betacellulin (BTC) purified from mouse β cell tumor (βTC-3) is a new member of the epidermal growth factor (EGF) family which can bind receptor tyrosine kinase, EGF receptor (erbB1) and erbB4. It has been demonstrated that proBTC mRNA was abundantly expressed in human pancreas tissue, and that BTC converted amylase-secreting rat acinar cell line (AR42J) into insulin- secreting cells, suggesting that BTC might be important for the growth and/or differentiation of islet cells. However, the cell type producing BTC in the pancreas has not been clarified. In this study, we examined the localization of BTC in human pancreas and islet cell tumors. Immunohistochemistry using specific antibodies to human BTC revealed that this protein was produced in a cells and duct cells, and probably in β cells in normal adult pancreas. Furthermore, strong immunoreactivity to BTC was detected in primitive duct cells of the fetal pancreas, and both insulinoma and glucagonoma cells also showed positive immunoreactivity to BTC. EGF receptor (erbB1) and erbB4 were expressed mainly in islet and duet cells, and duct cells, respectively. These results demonstrate the localization of BTC and its receptors, and suggest that BTC may be one of the factors that have physiologically important roles such as growth and differentiation of islet cells in the human pancreas.