The key for achieving a better outcome in cancer patients is a better understanding of cancer's molecular biology. In this master's thesis we investigated the proteolytic interaction of the uPAR protein and extracellular cathepsins, which could play a role in cancer progression and metastasis. To determine the cathepsin cleavage site on the uPAR protein, the cDNA fragment for uPAR was cloned and the reporter peptide was added to it. The fusion protein was expressed in MDA-MB-231, HEK-293 and HeLa human cell lines. The transfected HeLa cells, which were expressing the most uPAR of all tested cell lines, were treated with cysteine cathepsins K, L and S. We determined that all the cathepsins cleaved uPAR's ectodomain, with the most active cathepsin being cathepsin S. Via a tandem mass spectrometric analysis, we determined cathepsin S cleavage sites D299 and Q301 on the uPAR. The cleavage sites are in accordance with ectodomains masses obtained by immunological detection. According to uPAR's primary structure, the cleavage sites are in accordance with cathepsin S's proteolytic specificity and according to uPAR's tertiary structure, the cleavage sites are sterically accessible.