Immune globulin, prepared as a chemically and enzymatically unmodified solution in 10 percent maltose at pH 4.25, was administered intravenously, at a dose of 0.4 g per kg per day for five consecutive days, to seven alloimmunized patients who had acute nonlymphocytic leukemia. All patients had an approximately threefold rise in IgG level. Five patients showed no change in lymphocytotoxic antibody (LCTAb) activity and no response to random donor platelets that were administered after the immune globulin. The activity of LCTAb either disappeared or diminished after immune globulin infusion in two patients, and they had borderline acceptable corrected count increments 1 hour after transfusion of pooled random-donor platelet transfusions. Because of the diminished LCTAb activity, the improvements in response to pooled random-donor platelet transfusions cannot be ascribed to the administration of immune globulin. High-dose intravenous immune globulin has not been shown to be effective in reversing the effects of alloimmunization.