Carcinogenesis in the human colon is associated with a marked increase in the tissue content of the urokinase-type plasminogen activator (u-PA). This study was performed to determine the type of cells responsible for the u-PA increase in carcinomas of the colon and in their precursor lesions, the adenomas, by immunohistological evaluation applying monoclonal antibody 3689 directed to the beta-chain of u-PA. Normal intestinal mucosa (n = 17) showed hardly any staining of u-PA, but some lamina propria cells were faintly positive. Carcinomas (n = 17) and adenomas (n = 16) showed a considerable and comparable staining intensity of u-PA in neoplastic columnar epithelial cells, and this staining was found to be diffuse and cytoplasmic. In a majority of the neoplastic tissues the u-PA staining was found to be patchy and not related to known risk markers of malignancy such as dysplasia in the adenomas, or to prognostic determinants such as Dukes' classification or differentiation in the carcinomas. The observation of strong u-PA positive lamina propria cells in adenomas but infrequently observed in normal mucosa and carcinomas was noteworthy. u-PA staining intensity of the tissue sections was found to correlate well with the u-PA antigen level in the tissue extracts determined by ELISA (r = 0.52, P = 0.0001) but poorly with the u-PA activity determined enzymatically (r = 0.28, P = 0.05). In conclusion, the u-PA increase in neoplasia of the human colon can be attributed to an increased diffuse cytoplasmic content of u-PA in neoplastic columnar epithelial cells.