We have examined the effect of two i.v. immunoglobulin preparations on the metabolic and functional activities of neutrophil granulocytes from the peripheral blood. Production of superoxide anion (O2-) by granulocytes was measured through superoxide dismutase inhibitable reduction of ferricytochrome C after incubation of cells for various times together with immunoglobulin (concentration ranging from 0.25 to 5.0 mg/ml). The results showed dose-dependent response of O2- production independent of the incubation time. Granulocytes containing ingested Staphylococcus aureus released a significantly (P less than 0.001) larger amount of O2- and killed a higher number (P less than 0.001) of viable bacteria in the presence of 5 mg/ml immunoglobulin than did cells incubated in the absence of extracellular i.v. immunoglobulin. These data raise the possibility that immunoglobulin concentrates for i.v. use may enhance the anti-bacterial activities of phagocytic cells through direct stimulation of the respiratory burst. Inflammatory reactions observed during i.v. immunoglobulin infusion in hypo- or agammaglobulinaemic patients may also be related to phagocytic cell activation.