Expression of pancreatic secretory trypsin inhibitor (PSTI) gene was examined by Northern blotting analyses in 31 human colorectal tumors that included two benign adenomas and 26 adenocarcinomas. Among the total of 28 cases which proved to be adequate for mRNA analyses, all but one showed the expression of PSTI at various levels. In contrast, PSTI expression was not detected in two malignant lymphomas of the rectum. The level of PSTI expression was not correlated with the patient's age, sex, tumor location or size, stage of differentiation, lymph node metastasis, or progression stage. Some colorectal adenocarcinomas were also shown to express genes that can hybridize with human trypsinogen cDNA probe. It looks as though in these tumors, a protease(s) and its inhibitor are produced simultaneously as part of a cellular self-defense mechanism.