Intravenous gammaglobulin (IV-IgG) was administered to a patient with chronic idiopathic thrombocytopenic purpura due to an unusual IgM platelet autoantibody causing in vitro complement-dependent thrombocytoxicity and in vivo intravascular platelet destruction. After IV-IgG infusion the peripheral platelet count temporarily increased to normal values, the mean platelet survival time increased and the platelet sequestration pattern changed from intravascular to predominantly hepatic destruction. In vivo and in vitro observations in this patient illustrate a transient beneficial effect of gammaglobulin infusion due to interference with the complement-fixing autoantibodies against platelets.