Autoimmune (idiopathic) thrombocytopenic purpura (ITP) is a disease caused by autoantibodies directed against platelets, but the demonstration of specific antibodies has been difficult for a variety of reasons. In general, when the antibodies can be demonstrated, there is an inverse correlation with the platelet count. We have set up two assays, one a screening assay and one an assay for specific platelet glycoproteins, to aid in the diagnosis and treatment monitoring of patients with ITP. We will use the tests particularly for the followup of patients before and after treatment in a pilot study with NHLBI in the treatment setting of T-cell-depleted auto-stem cell transplantation in patients with severe ITP. Sixteen patients have been studied. A verbal presentation was given at the national meeting of the American Society of Hematology in December 1999.