Cathepsins, which are secreted by tumour and/or stromal cells, are thought to be involved in the degradative processes of tumour invasion and metastasis. The purpose of our study was to compare the cytosolic content of cathepsin B, L, and D in a series of matched malignant and adjacent normal colorectal tissues. Further we attempted to correlate these different proteinase values to classical clinico-pathological prognostic variables. Cathepsin B, L, and D activities were higher in tumour tissues than in normal mucosa (P < 10(-6), P < 0.004, P < 0.004, respectively) with median tumour/normal ratios of 7.9, 5.9, and 1.4, respectively. We found no difference in cathepsin B, L, and D activities either as a function of gender (except for cathepsin B values), age at time of surgery, tumour site, tumour differentiation, tumour stage (TNM or Astler-Coller staging system) or whether or not we found a mucinous component. Based on our data, cathepsin B seems to be the most discriminant parameter of the three proteinases that we studied, suggesting that cathepsin B expression may be of critical value in the progression of colorectal cancers.