SummaryAnti‐Rho(D) immunoglobulin (anti‐D) contained more high molecular weight (HMW) IgG polymers than intravenous non‐specific immunoglobulin (i.v. Ig). The low‐dose anti‐D and high‐dose i.v. Ig regimens used to treat idiopathic thrombocytopenic purpura (ITP) therefore contained similar total amounts of HMW IgG.In vitro, the HMW IgG polymers were more effective competitive inhibitors of monocyte phagocyte Fc receptors than monomeric IgG. The IgG subclass composition of anti‐D and i.v. Ig were both similar to normal human plasma. We conclude that the HMW IgG content but not the IgG subclass composition of anti‐D may explain its low‐dose therapeuti