Immunohistochemical demonstration of pancreatic secretory trypsin inhibitor in normal and neoplastic colonic mucosa. Specimens of normal and neoplastic colonic mucosa from 52 patients were analysed by immunohistochemistry using a monospecific polyclonal antiserum against human pancreatic secretory trypsin inhibitor (PSTI). In normal colonic mucosa PSTI was found in the goblet cells in the basal parts of the crypts. In adenomas of tubular, villous, and tubulo-villous types PSTI was also found in the upper parts of the polyps, usually occurring in the regeneration zone. There was a more intense staining reaction in polyps with increased atypia. Carcinomas of different types and of various grades of differentiation and of in situ type did not contain PSTI. These findings indicate that PSTI could be a marker for adenomatous rather than carcinomatous epithelium in the colon. Furthermore, the absence of the inhibitor in malignant cells might facilitate tissue invasion by malignant cells because of deficient protease inhibition. 