A rapid and sensitive immunofluorescence assay utilizing the flow cytometry to detect in platelet associated IgG (PAIgG) and platelet binding IgG (PBIgG) in patients with idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus (SLE) and leukemia cases received multiple platelet transfusion are described. Using this assay the percentages of PAIgG positive platelets of normal individuals were 2.7%. Increased levels were found in 22 cases of 23 patients with ITP in a range of 2.5-86.6%. The correlation of platelet counts and the percentages of PAIgG positive platelets was found to be significant (p<0.01). Elevated levels of platelet associated complement C3 (PAC3) in 21 out of 23 and platelet associated IgM (PAIgM) in 10 out of 11 were also found. Between PAIgG and PAC3, PAIgM, the good correlation were demonstrable (r=0.833, r=0.900). In SLE cases, the percentages of PBIgG and PBC3 positive platelets were elevated as well as in ITP cases, but PAIgG levels were decreased compared with ITP cases. Further, this assay was useful to predict the effects of platelet transfusion. Finally, in comparsion to other PAIgG assays, this new assays provide specific information in the nature of the immunoglobulin deposited on the platelet surface in ITP cases.