Studies have shown platelet-associated IgG (PAIgG) is abnormally elevated in idiopathic thrombocytopenic purpura (ITP) and also in thrombocytopenic disorders that are not thought to be immune-mediated. In ITP, PAIgG on the platelet surface is thought to be anti-platelet autoantibody, however the nature of IgG on the surface of platelets of patients with non-immune thrombocytopenia (NIT) is not known. To further investigate these issues we utilized confocal (LSCM) and electron microscopy (EM) to evaluate platelet morphology and the subcellular distribution of IgG in ITP and NIT platelets. Additionally patterns of IgG distribution were compared to flow cytometric measurements of IgG on permeabilized and non-permeabilized platelets, and in solubilized membrane and supernatant fractions of disrupted platelets using PAIgG phase II assays. EM analysis indicated ITP platelets (n=6; 327 analyzed pits) and NIT platelets (n=6; 260 analyzed pits) are morphologically indistinguishable from normal platelets (n=6; 360 analyzed pits) in their ultrastructural organization, cross-sectioned area, and number of organelles. As a result, the elevated total PAIgG in thrombocytopenic patients could not be attributed to alterations in the physical characteristics of platelets, as ITP and NIT platelets had a normal platelet size and a normal number of storage granules. Immunogold labeling of IgG in ITP platelets was localized primarily within a-granules with smaller amounts of IgG on their platelet surface and in their OCS. In comparison, PAIgG in NIT platelets was localized almost entirely within a-granules, with an absence of IgG on their platelet surface or within their OCS. LSCM demonstrated a granular pattern of IgG staining within ITP platelets and a rim pattern of IgG staining on their platelet surface (n=21 ). In comparison, PAIgG staining of NIT platelets was localized almost entirely within platelets, with an absence of IgG on the platelet surface (n=19). Flow cytometric measurement of PAIgG demonstrated elevated amounts of IgG within ITP (n=21) and NIT platelets (n=I9), with increased amounts of IgG on the surface of ITP platelets alone. Measurement of IgG in isolated membrane fractions demonstrated similar results, with elevated amounts of IgG in the membrane fractions of ITP platelets alone. The results of this study demonstrate that while both ITP and NIT have abnormal elevations of total PAIgG, ITP platelets alone have increased amounts of IgG on their membrane surface. We suggest that these differences in surface PAIgG distribution in ITP and NIT may be relevant to differences in platelet clearance in these disorders.