Matrix metalloproteinase-9 (MMP-9) may play a critical catalytic role in tissue remodeling in vivo, but it is secreted by cells as a stable, inactive zymogen, pro-MMP-9, and requires activation for catalytic function. A number of proteolytic enzymes activate pro-MMP-9 in vitro, but the natural activator(s) of MMP-9 is unknown. To examine MMP-9 activation in a cellular setting we employed cultures of human tumor cells (MDA-MB-231 breast carcinoma cells) that were induced to produce MMP-9 over a 200-fold concentration range (0.03-8.1 nM). The levels of tissue inhibitors of metalloproteinase (TIMPs) in the induced cultures remain relatively constant at 1-4 nM. Quantitation of the zymogen/active enzyme status of MMP-9 in the MDA-MB-231 cultures indicates that even in the presence of potential activators, the molar ratio of endogenous MMP-9 to TIMP dictates whether pro-MMP-9 activation can progress. When the MMP-9/TIMP ratio exceeds 1.0, MMP-9 activation progresses, but through an interacting protease cascade involving plasmin and stromelysin 1 (MMP-3). Plasmin, generated by the endogenous urokinase-type plasminogen activator, is not an efficient activator of pro-MMP-9, neither the secreted pro-MMP-9 nor the very low levels of pro-MMP-9 associated with intact cells. Although plasmin can proteolytically process pro-MMP-9, this limited action does not yield an enzymatically active MMP-9, nor does it cause the MMP-9 to be more susceptible to activation. Plasmin, however, is very efficient at generating active MMP-3 (stromelysin-1) from exogenously added pro-MMP-3. The activated MMP-3 becomes a potent activator of the 92-kDa pro-MMP-9, yielding an 82-kDa species that is enzymatically active in solution and represents up to 50-75% conversion of the zymogen. The activated MMP-9 enhances the invasive phenotype of the cultured cells as their ability to both degrade extracellular matrix and transverse basement membrane is significantly increased following zymogen activation. That this enhanced tissue remodelling capability is due to the activation of MMP-9 is demonstrated through the use of a specific anti-MMP-9 blocking monoclonal antibody.