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There has been some disagreement as to the clinical effect of intravenous immunoglobulin (IVIG) therapy on neonatal bacterial infections. We therefore evaluated the effect of IVIG therapy on neonatal polymorphonuclear leucocyte(PMN) functions by monitoring phagocytosis and hydrogen peroxide(H2O2) production. Subjects were 10 mature neonates who had normal plasma levels (i.e. equal to adult plasma levels) of IgG and nine premature neonates who had lower plasma levels of IgG. Phagocytosis by PMN was measured using flow cytometric analysis of whole blood. Addition ofγ-globulin to the whole blood of mature neonates increased phagocytosis, but not significantly. Higher doses of added γ-globulin(>2·0 mg/ml, the concentration was expressed as that in the final reaction volume) decreased phagocytosis to under baseline level. In premature neonates addition of γ-globulin increased phagocytosis and the significant maximum effect was observed with 0·5 mg/ml of the additional γ-globulin. Higher doses of additional γ-globulin(2·5 mg/ml) decreased phagocytosis to baseline level. Phagocytosis in four mature and four premature neonates was compared before and after 1 g/kg of IVIG therapy. Phagocytosis in mature neonates after IVIG therapy did not change compared with the pretreatment level. On the other hand, phagocytosis in premature neonates after IVIG therapy significantly increased compared with its pretreatment level. In both mature and premature neonates H2O2 production following phagocytosis varied in parallel with changes of phagocytosis. The patterns of H2O2 production following phagocytosis were essentially similar to those observed with phagocytosis. The above results are expected to form the basis for a rational indication for IVIG therapy against bacterial infections in neonates with low plasma IgG levels.