Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder characterized by thrombocytopenia due to autoantibody-induced platelet destruction. The majority of these autoantibodies are directed to epitopes on either glycoprotein (GP) IIb-IIIa or GPIb-IX. The newer antigen-specific autoantibody assays are capable of detecting both platelet-associated and plasma autoantibodies and have a definite role in the diagnosis of immune thrombocytopenia. A positive assay provides strong evidence for the presence of immune thrombocytopenia both in chronic ITP and in other diseases where immune thrombocytopenia may occur, such as collagen vascular disease and lymphoproliferative disorders. However, a negative assay does not rule out the presence of ITP. Somewhat concerning is the large number of patients who have negative assays. Several possible explanations for these observations are discussed. Recent studies have localized some ITP autoepitopes to specific regions of GPIIb-IIIa and GPIb-IX. Most autoepitopes on GPIIb-IIIa are conformational, in view of their dependence on divalent cations, and are localized to the N-terminal portion of GPIIb, while the GPIb-IX autoepitopes that have been identified are localized to GPIb amino acids 333-341.