Interactions of components of the fibrinolytic system with fibrin play a key role in localization and regulation of plasminogen activation at the clot surface. Although interaction of tissue plasminogen activator (tPA) with fibrin is well established, the prevailing view is that urokinase-type plasminogen activator (uPA) does not bind to fibrin. In this report, I show that although there is little or no interaction of single-chain urokinase (Sc-uPA), two-chain urokinase (Tc-uPA), or low molecular weight urokinase (LMW-uPA) with fibrin in the absence of divalent metal cations, Sc-uPA is effectively bound to fibrin in the presence of Zn2+. By comparison, Tc-uPA is poorly bound and LMW-uPA is not bound to fibrin in the presence of the metal ion. The Zn(2+)-mediated fibrin binding of Sc-uPA is reversible, specific, and saturable. The interaction involves a single class of binding site with dissociation constant of 0.3 microM and stoichiometry of 2.7. Lacking apparent affinity for fibrin, uPA has not been considered to play a role in physiological fibrinolysis. The present study conclusively shows that Sc-uPA possesses a latent affinity for fibrin that is expressed in the presence of Zn2+. These findings raise the possibility that Zn2+ plays a regulatory role in uPA-mediated fibrinolysis by promoting effective localization of Sc-uPA at the clot surface.