High‐dose intravenous immune globulin (IVIG) exerts a beneficial effect in a variety of immune disorders. One possible underlying mechanism of this effect could be interference with the complement system. This conclusion was based on the results obtained in animal models of complement‐mediated pathology, in vitro complement assays and studies on related human diseases. Clearance of IgM‐sensitized erythrocytes was specifically suppressed hy IVIG treatment. The same therapy prevented pulmonary endothelial cell lesions, the hallmark of Forssman shock, in 75% of animals. All control animals, either untreated or injected with control reagents, died within minutes following induction of Forssman shock. In vitro uptake of C3b and C4b complement fragments on to corpusculate immune complexes was significantly inhibited by IVIG. Studies that involved patients suffering from disorders with pathogenesis similar to animal models of complement‐mediated immune injury fully supported the hypothesis that IVIG interacts with activated complement components and prevents their deposition on target cells. The author's results suggest that IVlG can he an effective modulator of inappropriate complement attack.