Drug-induced immune thrombocytopenia (DITP) is caused by drug-dependent antibodies (DDAbs) that are nonreactive in themselves but bind tightly to specific platelet membrane glycoproteins (GP) when soluble drug is present at pharmacologic concentrations. This reaction takes place without covalent linkage of drug to the target, indicating that drug does not function as a classical hapten to promote antibody binding. Studies to define other mechanism(s) responsible for this interaction have been frustrated by the polyclonal nature of human DDAbs and limited quantities of antibody usually available. We produced 2 monoclonal antibodies (mAbs), 314.1 and 314.3, from a mouse immunized with purified human GPIIb/IIIa and quinine that recognize the N terminus of the GPIIb β propeller domain only when soluble quinine is present. Both monoclonals closely mimic the behavior of antibodies from patients with quinine-induced immune thrombo-cytopenia in their reactions at various concentrations of quinine and quinine congeners. Sequencing studies showed that the 2 mAbs are closely related structurally and that mAb 314.3 probably evolved from mAb 314.1 in the course of the immune response. These monoclonal reagents are the first of their kind and should facilitate studies to define the molecular basis for drug-dependent antibody binding and platelet destruction in DITP.