Heparanase is a mammalian endo-beta-D-glucuronidase that cleaves heparan sulfate side chains at a limited number of sites. Such enzymatic activity is thought to participate in degradation and remodeling of the extracellular matrix and to facilitate cell invasion associated with tumor metastasis, angiogenesis and inflammation. Traditionally, heparanase activity was well correlated with the metastatic potential of a large number of tumor-derived cell types. More recently, heparanase upregulation has been documented in an increasing number of primary human tumors, correlating with poor postoperative survival and increased tumor vascularity. Here, we employed anti-heparanase 733 polyclonal antibody that preferentially recognizes the 50 kDa active heparanase subunit over the 65 kDa proenzyme, as well as anti-heparanase 92.4 monoclonal antibody that recognizes both the latent and the active enzyme, to follow heparanase expression, processing and localization throughout the adenoma-carcinoma transition of the colon epithelium. Normal (nondysplastic) mucosa of the large bowel near epithelial neoplasms, as well as areas of mild dysplasia in adenomas, exhibited a strong reactivity with antibody 733 that became even stronger in foci of moderate dysplasia. Interestingly, although reactivity with antibody 733 was markedly reduced in severe dysplasia and in colorectal carcinoma, response to antibody 92.4 exhibited the opposite trend and staining intensities increased in parallel with tumor stage, the highest being in carcinoma cells. Involvement of latent heparanase (detected by 92.4, but not by 733 antibody) in tumor progression was suggested by activation of the Akt/PKB signal transduction pathway upon heparanase overexpression or exogenous addition to HT29 human colon carcinoma cells. These results suggest that heparanase expression is induced during colon carcinogenesis, and that its processing, conformation and localization are tightly regulated during the course of colon adenoma-carcinoma progression.