Interaction of the platelet receptor glycoprotein (GP) Ib-V-IX with von Willebrand factor exposed at a site of vascular injury is an essential step in the initiation of a hemostatic plug. Proteolytic cleavage (shedding) of the GPIbα subunit was first described >25 years ago, the protease mediating this event as well as its physiological function, however, have not been elucidated. We reported recently that shedding of GPIbα induced by platelet storage or mitochondrial injury involves a platelet-derived metalloproteinase(s). Here we show that GPIbα shedding in response to mitochondrial injury or physiological activation is inhibited in platelets obtained from chimeric mice, which express inactive tumor necrosis factor-α converting enzyme (TACE^ΔZn/ΔZn) in blood cells only. Shedding was also inhibited in mouse and human platelets in the presence of 2 potent TACE inhibitors: TAP1 and TMI-1. Our data further suggest that TACE is important in the regulation of GPIbα expression in vivo because we observed an ≈90% reduction in soluble GPIbα (glycocalicin) in plasma of TACE^ΔZn/ΔZn chimeras as well as significantly increased levels of GPIbα on circulating platelets. In contrast, shedding of P-selectin from activated platelets was not affected by the mutation in TACE. Damaged TACE^ΔZn/ΔZn platelets were further characterized by a markedly improved post-transfusion recovery and hemostatic function in mice. In conclusion, our data demonstrate that TACE is expressed in platelets and that it is the key enzyme mediating shedding of GPIbα.