T hrombocytopenia and thrombosis following treatment with the integrin αIIbβ3 antagonist eptifibatide are rare complications caused by patient antibodies specific for ligand-occupied αIIbβ3. Whether such antibodies induce platelet clearance by simple opsonization, by inducing mild platelet activation, or both is poorly understood. To gain insight into the mechanism by which eptifibatide-dependent antibodies initiate platelet clearance, we incubated normal human platelets with patient serum containing an αIIbβ3-specific, eptifibatide-dependent antibody. We observed that in the presence of eptifibatide, patient IgG induced platelet secretion and aggregation as well as tyrosine phosphorylation of the integrin β3 cytoplasmic domain, the platelet FcγRIIa Fc receptor, the protein-tyrosine kinase Syk, and phospholipase Cγ2. Each activation event was inhibited by preincubation of the platelets with Fab fragments of the FcγRIIa-specific mAb IV.3 or with the Src family kinase inhibitor PP2. Patient serum plus eptifibatide did not, however, activate platelets from a patient with a variant form of Glanzmann thrombasthenia that expressed normal levels of FcγRIIa and the αIIbβ3 complex but lacked most of the β3 cytoplasmic domain. Taken together, these data suggest a novel mechanism whereby eptifibatide-dependent antibodies engage the integrin β3 subunit such that FcγRIIa and its downstream signaling components become activated, resulting in thrombocytopenia and a predisposition to thrombosis.