Pancreatic secretory trypsin inhibitor (PSTI) is a potent serine protease inhibitor that prevents excessive digestion of the gastrointestinal mucus but may also directly affect epithelial function. We therefore examined the distribution of PSTI in the human adult and fetus using immunohistochemistry and in situ hybridization and examined its effects on cell proliferation and migration in vitro. PSTI peptide and mRNA were found in the exocrine pancreas, mucus-producing cells of the normal gastrointestinal tract, acinar component of the normal breast, and surface epithelial cells at the edge of benign gastric ulcers. Peptide, but not message, was identified in the renal proximal tubule, probably reflecting reabsorption of filtered peptide. Purified human PSTI did not affect proliferation of the human colonic cell line HT-29 but caused a threefold increase in the rate of migration in an in vitro wounding model of restitution. This effect was inhibited by co-administering a PSTI-neutralizing antibody, a transforming growth factor-beta-neutralizing antibody, or an epidermal growth factor receptor-blocking antibody. As PSTI is widely distributed in several human organ systems and stimulates cell migration in vitro, we conclude that PSTI is likely to have additional roles to that of preserving the gastrointestinal mucous layer from excessive digestion.