Integrin αIIbβ3 plays a pivotal role in hemostasis and thrombosis by mediating platelet adhesion and platelet aggregation. Integrin αIIbβ3 contains an on/off switch that regulates its ligand binding affinity. The switch from “off” to “on” is commonly referred to as integrin activation. We recently identified a redox site within the extracellular domain of the platelet integrin αIIbβ3 that exhibits many properties that one might expect of the on/off switch [Yan, B., and Smith, J. W. (2000) J. Biol. Chem. 275, 39964−39972]. Several independent reports show that reducing agents, such as dithiothreitol, can activate integrins. The objective of the present study was to determine if the effects of DTT can be attributed to a perturbation at the integrin redox site. Indeed, we find that DTT reduces two disulfide bonds within the integrin's cysteine-rich domain. Such bond reduction leads to global conformational changes within both αIIb and β3 and the opening of the RGD and fibrinogen binding sites. These findings causally link the reduction of disulfide bonds within the integrin's redox site to transitions in the integrin's activation state.