Plasminogen is a single-chain glycoprotein found in human plasma and extracellular fluid. Certain activators, such as tissue plasminogen activator (tPA), convert plasminogen to its active form, plasmin. Plasminogen has been used to study its conversion into plasmin.

Plasminogen is the inactive precursor of the protease plasmin. Plasminogen is activated by the action of either tissue plasminogen activator (tPA), which primarily activates the fibrinolytic (thrombolytic) activity of plasmin, or urokinase plasminogen activator (uPA), which is associated with extracellular matrix remodeling and cell migration. Plasmin cleaves fibrin/fibrinogen and blood coagulation factors V/Va and VIII/VIIIa. It activates matrix metalloproteinases by cleaving the inactive proenzymes. It is also involved in the activation of some growth factors, such as vascular endothelial growth factor (VEGF) and transforming growth factor β (TGF-β).

The amount of enzyme which will hydrolyse 1 µmol of N-tosyl-L-arginine ester in 30 minutes at 37 °C at pH 8,0.