After an in vitro incubation of platelets with fibrillar collagen, their elastase activity is markedly and rapidly increased while proelastase decrease: proelastase is activated in situ into elastase which is released in its active form from the platelet. The activation of proelastase is likely due to the action of a trypsin-like enzyme present in the platelet. This protease has the same type of localization as proelastase and elastase: their highest activity is associated with light granules but part of these enzymes (or precursor) is also associated with the membranes. The mechanism of the arterial elastolysis induced by the platelets probably involves their adhesion to intimal thrombogenic surfaces (collagen) followed by a reaction during which proelastase would become available to the trypsin-like enzyme and would be activated into elastase directly released in the vessel wall.