The primary structure of a human pancreastatin-like peptide was determined from a pancreatic glucagonoma. The 28-amino acid peptide was identified using a specific antibody raised against porcine pancreastatin 1–49 and showed a 75% sequence homology with porcine pancreastatin 22–49 and bovine chromogranin A 267–294. Several forms of pancreastatin-like immunoreactivity were found in human endocrine tumors of which the purified peptide was the smallest and contained the active sequence of pancreastatin.