Although considerable knowledge has been acquired concerning the chemical nature of insulin and its pathway of biosynthesis, relatively little is known about the other proteins which constitute the insulin secretory granule. This, in part, has been a consequence of the difficulties of working with pancreatic islets as a tissue source and the attendant problems of obtaining sufficient material of suitable purity for experimental purposes. The scope for further investigation is perhaps best illustrated by the fact that of the 150 or so insulin granule proteins which may be visualized by electrophoresis the identity of less than 10 has been established. It is clear that some of the proteins will be shared by other granule types in accordance with the common cellular pathways involved in granule morphogenesis and exocytotic secretion. Equally though, it is also apparent that many of the peptides of the insulin granule are tailored for specific functions within the B-cell.