The precise pathogenic mechanism of platelet destruction in immune thrombocytopenias is not known, although many investigators have found that platelet-associated IgG is increased in these diseases. We report here the differentiation between specific binding of anti-platelet antibody, associated with platelet destruction, and the ubiquitous presence of nonspecific, platelet-associated IgG. Using an electrophoretic separation and antibody overlay technique, we have identified a specific membrane protein that bears target platelet antigens in immune thrombocytopenias. When posttransfusion purpura serum was studied, antibody binding to the PlA1 antigen on glycoprotein IIIa was readily distinguished from the nonspecific binding of immunoglobulin to a protein of 200,000 mol wt. After reduction of disulfide bonds, the PlA1 antigenicity was not observed, and IgG bound nonspecifically to a protein band with an apparent molecular weight of 45,000. We have also identified anti-platelet antibodies in patients with idiopathic thrombocytopenic purpura and determined their antigenic specificity. Antibodies which bind to a 100,000-mol wt protein were found in nine of thirteen patients with chronic disease. The antigens in three of these cases were studied in detail by using both reduced and nonreduced control and Glanzmann's thrombasthenic platelets. Target antigens were localized to glycoprotein IIIa, but are different from PlA1. The immune thrombocytopenic purpura antigenic system is clearly distinguished from nonspecific platelet-associated IgG. Sera from eight children with acute idiopathic thrombocytopenic purpura were also studied. In all cases, the nonspecific IgG binding to the 200,000-mol wt protein was observed. However, we were unable to demonstrate antibody binding to glycoprotein IIIa, which suggested that the acute childhood form of this disease may have a different pathogenic mechanism than that of the autoimmune chronic cases.