AbstractThis study investigates the dose‐dependent effect of administration of commercially available intravenous immunoglobulins (IVIG) on the survival of newborn rats experimentally infected with group B streptococci (GBS). The opsonic activity of various concentrations of IVIGin vitrowas determined by examination of opsonophagocytosis of GBS by peritoneal macrophages and bacterial killing by blood neutrophils.The best survival was observed in newborn rats who received immunoglobulins at doses of 250, 500 and 1000 mg/kg of bodyweight. The survival of animals that received either 125 mg/kg or 2.5 g/kg of immunoglobulins was no better than that of animals who received no immunoglobulins. The maximal phagocytosis and bacterial killing were observed atin vitroimmunoglobulin concentrations ranging from 32 to 250 mg/dL (thesein vitroconcentrations may correspond roughly toin vivoadministration doses ranging from 150 to 1200 mg/kg). These doses are comparable to the doses of IVIG currently administered to neonates. However use of very high doses may be harmful to babies since the high concentration of non‐specific immunoglobulins inhibitedin vitrophagocytosis and bacterial killing by phagocy