Plasminogen, tethered to the cell surface by its association with histidine-rich glycoprotein, is cleaved and activated to plasmin by the action of urokinase plasminogen activator bound to uPAR, its cell-surface receptor. The association of both substrate and enzyme with the cell surface is necessary for the reaction to proceed efficiently (Ellis et al. 1991). While the one-chain form of urokinase plasminogen activator is lower than that of the two-chain form, it is still sufficient to initiate the process of plasmin activation (Ellis et al. 1989; Lijnen et al. 1986).