In chronic autoimmune thrombocytopenic purpura (AITP), an indirect monoclonal antibody immobilised platelet antigen (MAIPA) assay detected serum glycoprotein (GP) Ilb/IIIa antibodies in 16/39 (41%) cases. In patients with clinically active AITP, a direct MAIPA assay detected platelet-associated GP Ilb/IIIa kantibodies in 8/13 (62%) cases. Platelet bound and serum antibody concentrations suggested a high antibody affinity for platelet membrane glycoprotein Ilb/IIIa. Five AITP patients with platelet associated glycoprotein Ilb/IIIa antibodies were treated with intravenous anti D immunoglobulin. All showed an increase in platelet counts and a decrease in platelet associated autoantibody. These responses could be due to immunosuppressive anti-idiotype antibodies in anti D immunoglobulin.