A prospective randomized study comparing 7S immunoglobulin G to a 5S IgG preparation for therapy of acute idiopathic thrombocytopenic purpura was conducted. The 5S preparation differed from the 7S preparation in that it lacked part of the Fc portion of the IgG molecule. Both groups were given 400 mg IgG/kg body weight over 5 days. All patients had platelet counts less than or equal to 30 X 10(9)/L before IgG infusion. Nine of the 10 patients in the 7S treatment group, compared with three of 10 patients in the 5S treatment group, responded to therapy with an increment in platelet counts greater than 100 X 10(9)/L within 4 days of completing the infusion treatment. Furthermore, the rate of increase of the platelets was more rapid in the 7S group. The results emphasize the efficacy of partial Fc for management of acute idiopathic thrombocytopenic purpura.