The maintenance of equilibrium between proteases and their inhibitors has substantial significance for the pathogenesis of the neoplastic disease. It concerns especially the invasiveness of the neoplasms, which increases with rising of the proteases activity or with the decrease of the inhibitory activity of antiproteases. The neoplastic cell has the faculty of producing numerous proteases which by damaging connective tissue facilitates the invasion by the neoplasm. Invasiveness of the neoplasms may be augmented by the production of proteases by the host cells stimulated by the neoplasm. The condition of the proteases--antiproteases balance is also influenced by the production of antiproteases by neoplastic cells. Among numerous antiproteases alpha-2-macroglobulin has most intensive cytotoxic action in relation with the neoplasm. According to data collected from the literature, antiproteases can be regarded as anticarcinogenic agents. On account of this, the estimation of the antiproteases level can be regarded as a measure of the ability of the body system to resist the invasiveness of neoplasms.