AbstractMany human cells and cell lines possess a specific receptor that binds urokinase plasminogen activator (uPA) with an affinity of about 10−10 M. Bound enzyme is not internalized, is slowly dissociated, and retains its enzymatic activity. The amino acid sequence of uPA responsible for receptor binding is located within the first 35 aminoterminal residues, ie, in the growth factor domain. Binding, however, is not competed for by other proteins that contain the growth factor domain (including epidermal growth factor). Cells that produce uPA secrete the pro‐uPA form, which subsequently binds to the receptor. A431 cells, in fact, have their receptors completely saturated with pro‐uPA. It is proposed that uPA:uPA‐receptor interaction plays a direct role in physiological and pathological processes that require cell migration.