Fibronectin (FN) is a modular glycoprotein encoded by a single gene. A soluble form of this protein is found in the plasma of several animals. Alternative splicing of pre-mRNA at three sites produces cellular and plasma FNs. The plasma form contributes to blood clotting and thrombosis. Many extracellular matrices (ECM) contain an FN network associated with a variety of cell activities through binding to cell surface integrin receptors. Fragments of FN can have cryptic activities that are specific to these fragments rather than to the intact protein. The metalloprotease activity present in the basement membrane and plasma fibronectins has been intensively studied in humans, bovine and rats. Organic inhibitors that are selective for the human cryptic enzyme have been produced.