Expression of the cysteine proteinase cathepsin B and its physiological inhibitor cystatin C was analyzed in vitro in 1 human fibrosarcoma and 4 human colon carcinoma cell lines. Cystatin C antigen as well as cathepsin B activity were detected in the conditioned media of the 5 cell lines. The corresponding cell extracts expressed high levels of cathepsin B activity, whereas only trace amounts of cystatin C antigen could be found. Northern-blot analysis revealed the presence in the 5 cell lines of a 0.8-kb cystatin C mRNA transcript and 2 cathepsin B transcripts of 2.3 and 4.3 kb. Pepsin treatment of tumor-cell-released cathepsin B induced an average 7.3-fold increase in activity, indicating that the enzyme was mainly present as a latent form in conditioned medium. The pepsin-activated cathepsin B from one colon carcinoma cell line was further characterized using the cysteine proteinase inhibitors E-64, recombinant cystatin C, a cystatin-C-derived peptidyl inhibitor (Z-LVG-CHN2), and cathepsin-B-specific diazomethyl ketone inhibitors (Z-FT(OBzl)-CHN2, Z-FS(OBzl)-CHN2). This activity was totally neutralized by recombinant cystatin C, suggesting a potential for interaction between released extracellular cathepsin B and cystatin C. In vitro assays of degradation of extracellular matrix showed that cysteine proteinase inhibitors could decrease matrix degradation induced by pepsin-activated conditioned media. With colon cells, this inhibition was not observed, indicating a requirement for an extracellular activation of latent cathepsin B. Our data provide evidence that cystatin C and latent cathepsin B are both released extracellularly by colon carcinoma cells in vitro. They suggest that cystatin C and cathepsin B interactions may participate, in an as yet unelucidated way, in the modulation of the invasive phenotype of human colonic tumors.