Modern data on participation of proteolytic enzymes from tissues and blood in malignant growth as well as their role in tumor invasiveness and metastatic spreading are reviewed. Alterations in content of the protein inhibitors of proteolysis occurred in tumors as well as diagnostic and prognostic significance of the inhibitors estimation in blood serum of oncological patients are considered. The experimental data on the antitumoral effect of native and synthetic inhibitors of proteolysis are discussed.