The potential for proteases to regulate mammalian TLR signalling is controversial. We found that inhibition of extracellular serine proteases did not reduce activation of TLR4, but observed that the protease plasmin, an important fibrinolytic plasma enzyme that also exerts proinflammatory functions in monocytes, potentiated TLR2 and TLR4 signalling in RAW264.7 macrophages. Plasmin enhanced endogenous production of TNF alpha and activation of an NF-kappa B reporter plasmid. These actions were prevented by inhibition of its proteolytic activity and were not recapitulated by agonists of protease-activated receptors. These Studies link fibrinolysis and TLR signalling, identifying further mechanisms potentially involved in activation of innate immunity. (c) 2006 Elsevier Inc. All rights reserved.