Integrins can signal upon binding of their ligand, presumably because of conformational changes induced by ligand-binding. It has been postulated that ligand binding causes changes in the affinity of the integrin to its ligand. In order to test for ligand-induced change in the affinity of platelet alpha2beta1 to collagen, labelled viable platelets were passaged through a column of fibrillar collagen and stringent lysis conditions were used to remove all low-affinity receptors. A high-affinity fraction left on the collagen could be eluted with dithiothreitol (DTT) and 2% Sodium dodecyl sulfate (SDS). Antibodies raised against this fraction, identified alpha2beta1 by Western-blotting. Functional tests performed with the antibodies confirmed the involvement of the high-affinity proteins in platelet-collagen interactions attributed to alpha2beta1: inhibition of collagen-specific platelet adhesion and aggregation. EDTA, chaotropic agents or low pH did not elute the high affinity fraction of alpha2beta1. However, DTT followed by acetic acid did, which indicates that the steps necessary to disrupt the high-affinity collagen alpha2beta1 bond are reduction of disulfide bond(s) followed by disruption of electrostatic interactions. Our data 2 1 suggest that (i) ligand binding induces the formation of a new disulfide bond in a fraction of alpha2beta1, (ii) that this bond is an intrareceptor, and (iii) that this change increases the affinity of the receptor to its ligand.