SummaryThe mode of action of high dose intravenous gammaglobulin (IVGG) in idiopathic thrombocytopenic purpura (ITP) is controversial. This study used immunofluorescence techniques to investigate the possibility that IVGG binds to peripheral blood cells and that such binding blocks attachment of cell specific autoantibodies. IVGG bound to platelets, neutrophils, monocytes and lymphocytes. Erythrocytes appeared to bind only anti‐A and anti‐B present in the IVGG. Multimeric and monomeric fractions of IVGG also bound to these cells. However, F(ab′)2fragments of the IVGG reacted only weakly with platelets, neutrophils, monocytes and lymphocytes. Binding of IVGG to platelets and neutrophils did not prevent opsonization of these cells by monoclonal antibodies with specificities corresponding to those previously ascribed to platelet and neutrophil autoantibodies. This suggests that IVGG does not act by blocking attachment of autoantibodies to platelets i