Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder due to autoantibodies against platelets that result in their destruction. In some patients, these autoantibodies bind to platelet glycoprotein (GP) IIIa. With the aim of better defining the antigenic epitopes, plasma from 13 selected patients with chronic ITP known to have anti-GPIIb/IIIa autoantibodies was tested for reactivity with nine synthetic peptides corresponding to different regions of the GP IIIa molecule. Of these plasmas, five bound significantly (P less than .001) with either peptide 8 (amino acids 721-744) or peptide 9 (amino acids 742-762), which together form most of the carboxyterminal region presumed to be the cytoplasmic domain. Three of these positive plasmas, were tested further. In two of these positive plasmas, the anti-peptide antibodies represented greater than 80% of the detectable circulating autoantibody. To further evaluate the importance of the carboxyterminal region as an antigenic site, the chronic ITP plasmas were tested against Chinese hamster ovary cells transfected with GPIIb and either whole GPIIIa or GPIIIa lacking amino acids 728 to 762. Ten of the 13 plasmas required the presence of this region for significant autoantibody binding. We conclude that the carboxyterminal region is an important area for stimulating antiplatelet autoantibody formation in some patients with chronic ITP. It is not known whether these autoantibodies to the presumed cytoplasmic domain play an important role in the pathogenesis of the disease or occur as a secondary phenomenon during the course of platelet destruction.