Many studies of malignant cells or tissues in culture have implicated cysteine proteases in the progression of malignancy. We have extended these observations by measuring quantitative and qualitative changes in the expression of cathepsin B-like and L-like cysteine proteases during the growth and development of human colorectal carcinomas. Data derived from matched pairs of normal colorectal mucosa and carcinoma tissue from 27 patients demonstrated that both cathepsin B-like and cathepsin L-like specific activities were significantly elevated (P less than 0.005) in the carcinoma tissue, while levels of endogenous cysteine protease inhibitor remained constant. Correlation of cathepsin enzyme activities with different stages of colorectal cancer demonstrated significantly higher cysteine protease activities in individuals with Dukes' A tumors (tumors confined to the bowel wall) than in patients with more advanced tumors (Dukes' B, C, or D tumors) (P less than 0.01-0.05). The relative proportion of activities contained in tumor epithelial and stromal elements remains to be elucidated. These results suggest an important role for cysteine proteases in the early progression of human colorectal carcinoma.