The urokinase-type plasminogen activator (uPA) and the matrix-degrading metalloproteinases MMP-2 and MMP-9 (type IV collagenases/gelatinases) have been implicated in a variety of invasive processes, including tumor invasion, metastasis and angiogenesis. MMP-2 and MMP-9 are secreted in the form of inactive zymogens that are activated extracellularly, a fundamental process for the control of their activity. The physiological mechanism(s) of gelatinase activation are still poorly understood; their comprehension may provide tools to control cell invasion. The data reported in this paper show multiple roles of the uPA-plasmin system in the control of gelatinase activity: (i) both gelatinases are associated with the cell surface; binding of uPA and plasmin(ogen) to the cell surface results in gelatinase activation without the action of other metallo- or acid proteinases; (ii) inhibition of uPA or plasminogen binding to the cell surface blocks gelatinase activation; (iii) in soluble phase plasmin degrades both gelatinases; and (iv) gelatinase activation and degradation occur in a dose- and time-dependent manner in the presence of physiological plasminogen and uPA concentrations. Thus, the uPA-plasmin system may represent a physiological mechanism for the control of gelatinase activity.