The effect of purified human immunoglobulin G, prepared for intravenous administration by polyethylene glycol precipitation and ion exchange chromatography, on polymorphonuclear leukocytes (PMN) in vitro was studied. IgG induced a dose-dependent stimulation of the oxidative metabolism of the cells, as measured by release of superoxide anion and by chemiluminescence. The lowest concentration giving a detectable effect was 0.001 mg/ml. Two commercially available preparations of IgG for clinical intravenous use caused similar stimulation of the PMN although they differ in methods of isolation and purification. The addition of 10% maltose markedly reduced the stimulation by all three preparations of IgG. We suggest that i.v. infusion of purified IgG may lead to activation in vivo of host phagocytic cells without participation of complement, and that this interaction may be responsible for the hitherto unexplained side-effects caused by i.v. infusion of such preparations.