4 children with idiopathic thrombocytopenic purpura were treated with high dose i.v. IgG. Platelet associated immunoproteins were determined with an Elisa technique. Platelet counts increased in all patients, while simultaneously platelet associated IgG, IgA, IgM, C3 and C4 decreased. Serum antiplatelet antibody increased during treatment suggesting that administration of high doses of IgG interferes with the binding of antiplatelet antibody.