Patient samples contain drug-dependent antibodies that immunoblot GPIIIa in the presence of quinine. Patients 1, 2, 3, 5, and 6 had IgG antibodies that recognized a protein corresponding to GPIIIa in the presence but not in the absence of 0.4 mM quinine (immunoblot against whole platelet lysate). GPIIIa is marked with monoclonal AP6 specific for amino acid residues 211-221. The prominent high-molecular-weight bands correspond to naturally occurring platelet IgG.