We have previously developed a radioimmunoassay to detect and quantitate IgG and C3 on platelets. We have applied this assay to studies of patients with ITP, post transfusion purpura and heparin associated thrombocytopenia. We are currently utilizing this assay to determine the incidence of immunologic remission in patients with ITP who are in spontaneous clinical remission as well as those patients in remission following corticosteroids or splenectomy. We are especially concerned with the risk of neonatal thrombocytopenia developing in women with clinically silent but immunologically active ITP. We are studying the correlation between the serologic status of these patients and the outcomes of their pregnancies. We are utilizing a modification of this assay to study the biochemistry of the auto-antigen recognized by antibodies occuring in adults with ITP, post viral ITP in children, ITP associated with SLE, CLL, etc.