A patient with chronic idiopathic thrombocytopenic purpura (ITP), a 28-year-old group A, Rh-positive woman, also had hemolytic anemia. At that time, her red cells typed as group A, but the serum typed as group O. This typing followed intravenous high-dose immune globulin therapy, (400 mg/kg for 5 successive days) and was attributable to A antibody of the IgG type, composed of IgG1, IgG2, IgG3, and IgG4, in the immune globulin preparation, which had been given for treatment of ITP. All lots of commercial human plasma preparation products (heat-treated human plasma protein, albumin, factor VIII or IX products, and immune globulin) examined were found to contain A and/or B antibodies of the IgM or IgG type, with titers of 1 to 1024. Twelve of the 20 lots of immune globulin contained RBC antibodies of the IgG type. Subclass IgG1 was found in all 12 of these lots, and all four subclasses, including IgG3, were detected in 8 lots. All 8 showed antibody titers higher than 64.