Since the discovery of fibronectin as a transformation-sensitive 'cell surface' protein, it has been the focus of intensive studies. Fibronectin has multiple interactions and functions and is composed of distinguishing properties of malignant cells, properties. Invasiveness and metastasis, distinguishing properties of malignant cells, involve penetration through components of the extracellular matrix. Enzymatic degradation of matrix components is involved in these phenomena. Proteolytic targets of the pericellular matrices of cells in culture include fibronectin that has been found to be even selectively susceptible to proteinases. Defined fragments of fibronectin have transformation-promoting activity in experimental conditions and such fragments, detected in tumor patient body fluids, may serve as markers for tumor progression.