Pancreatic acinar cells contribute to .beta.-cell neogenesis. Hamsters administered pentadecapeptide of INGAP protein demonstrated a 2-fold increase in the number of extra-islet acinar-associated .beta.-cell clusters resulting in a 2.8-fold increase in the overall extra-islet .beta. cell mass as compared to controls. In mammals acinar-to-.beta.-cell differentiation provides an alternate pathway to .beta.-cell neogenesis; INGAP peptide plays a significant role in this process.