Integrins not only bind adhesive ligands, they also act as signalling receptors. Both functions allow the integrin αIIbβ3 to mediate platelet aggregation. Platelet agonists activate αIIbβ3 (inside-out signalling) to allow the binding of soluble fibrinogen. Subsequent platelet aggregation leads to outside-in αIIbβ3 signalling, which results in calcium mobilization, tyrosine phosphorylation of numerous proteins^, including β3 itself, increased cytoskeletal reorganisation and further activation of αIIbβ3 (ref. ). Thus, outside-in signals enhance aggregation, although the mechanisms and functional consequences of specific signalling events remain unclear. Here we describe a mouse that expresses an αIIbβ3 in which the tyrosines in the integrin cytoplasmic tyrosine motif have been mutated to phenylalanines. These mice are selectively impaired in outside-in αIIbβ3 signalling, with defective aggregation and clot-retraction responses in vitro, and an in vivo bleeding defect which is characterized by a pronounced tendency to rebleed. These data provide evidence for an important role of outside-in signalling in platelet physiology. Furthermore, they identify the integrin cytoplasmic tyrosine motif as a key mediator of β-integrin signals and a potential target for new therapeutic agents.