Intravenous infusion of gammaglobulin (IVGG) has been shown to be effective in the maintenance treatment of children and adults with immune thrombocytopenic purpura (ITP). The objective of this treatment is not to increase the platelet count into the normal range but rather to keep the platelet count above a safe level, usually 20,000-30,000/microliters. This use requires periodic infusion whenever the platelet count falls. The dose of gammaglobulin in previous studies was 1 gm/kg/infusion in the majority of patients. This study compared doses of 0.5 and 1.0 gm/kg/infusion by alternating the two doses in the maintenance treatment of 11 patients with ITP. There was no significant difference in the duration of response following single IVGG infusion between these two doses. This finding suggests that there is no advantage to the greater dose of IVGG and will substantially lower the cost of as well as facilitating IVGG maintenance treatment. It was not possible to determine whether a lower dose would impact adversely on the curative potential of IVGG. This effect of IVGG has never been proved and, if it exists, may be mediated by a different mechanism from the acute platelet response.