INTRODUCTION: The regulation of extracellular proteolytic activity via the plasminogen activation system is complex, involving numerous activators, inhibitors and receptors. Previous studies on monocytic and colon cell lines suggest that plasmin pre treatment can increase plasminogen binding, allowing the active enzyme to generate binding sites for its precursor. Other studies have shown the importance of pre formed receptors such as annexin II heterotetramer. However, few studies have utilised techniques which exclusively characterise cell surface events and these mechanisms have not been investigated at the breast cancer cell surface. METHODS: We have studied plasminogen binding to MCF 7 in which uPAR levels were upregulated by PMA stimulation allowing flexible and transient modulation of cell surface uPA. Similar experiments were also performed using MDA MB 231, which overexpress uPAR/uPA endogenously. Using techniques which preserve cell integrity, we characterize the role of uPA as both a plasminogen receptor and activator and quantify the relative contribution of pre formed and cryptic plasminogen receptors to plasminogen binding. RESULTS: Cell surface plasminogen binding was significantly enhanced in the presence of elevated levels of uPA in an activity dependent manner and was greatly attenuated in the presence of the plasmin inhibitor aprotinin. Pre formed receptors were also found to contribute to increased plasminogen binding following PMA stimulation and co localise with uPA/uPAR and plasminogen. Nevertheless, a relatively modest increase in plasminogen binding capacity coupled with an increase in uPA lead to a dramatic increase in the proteolytic capacity of these cells. CONCLUSION: We show that the majority of lysine dependent plasminogen binding to breast cancer cells is ultimately regulated by plasmin activity and is dependent on the presence of significant levels of active uPA. The existence of a proteolytic positive feedback loop in plasminogen activation has profound implications for the ability of breast cancer cells expressing high amounts of uPA to accumulate a large proteolytic capacity at the cell surface, thereby conferring invasive potential.