Blood platelets limit blood loss at sites of vascular injury by forming a mechanical plug. They are also involved in thrombosis, atherosclerosis, inflammation and metastasis. Platelet activation is essential for these physiological and pathological reactions and depends upon their adhesion to the vessel wall and attachment to each other in the aggregation process. The two known pathways of aggregation are mediated by the release of endoperoxides/throm-boxane A₂ and ADP^1–3 which amplify platelet aggregation. Here we report the identification of a new pathway of aggregation which is mediated by the release of a metalloproteinase enzyme, gelatinase A.