Cathepsin B is a lysosomal cysteine proteinase that has the ability to degrade several extracellular matrix components at both neutral and acidic pH and has been implicated in the progression of several human and rodent tumors. We have studied the expression of cathepsin B in human colorectal tissues using a monospecific polyclonal rabbit antibody raised against human liver cathepsin B. In immunoblots of normal and neoplastic colorectal tissues this antibody specifically recognized only cathepsin B. We studied 101 cases of formalin-fixed, paraffin-embedded tissue (15 normal mucosa, 17 adenomas, and 69 carcinomas). Epithelial cells of normal mucosa and adenomas were either negative or showed a weak granular reactivity located in the paranuclear and apical cytoplasm of superficial cells. Small clusters of histiocytes were also positive in the region of the superficial area of the lamina propria. In carcinomas, increased expression of cathepsin B correlated with advanced stage of the disease. Increased immunoreactivity of cathepsin B in malignant cells was associated with either a diffuse cytoplasmic staining or was polarized to the basal pole of the cells. This is in contrast to the punctate paranuclear staining pattern observed in normal colonic mucosal cells. In tumor stromal cells, increased expression of the enzyme correlated with neoplastic progression. Expression of high levels of cathepsin B in the tumor epithelial cells was associated with a significantly shorter survival of the patients. In conclusion, our results indicate that cathepsin B expression is up-regulated in human colorectal carcinomas compared with normal mucosa and adenomas and correlates with tumor progression.