Abstract: Proliferating pancreatic islet cells obtained by the method of isolating a population of cells that preferably includes predominantly islet precursor cells that express one or more marker associated with an islet precursor cell and providing the precursor cells with one or more a pancreatic differentiation agent so that a population of cells is obtained that has a high proportion of cells with phenotypic characteristics of functional pancreatic islet ?-cells. Optionally, the precursor cells are pretreated by providing them with one or more cell expansion agent to increase the number of cells in the population prior to differentiation. The pancreatic differentiation agent composition comprises a gastrin/CCK receptor ligand, e.g., a gastrin, in an amount sufficient to effect differentiation of pancreatic islet precursor cells to mature insulin-secreting cells.