The functional antibacterial activity of an intravenous immunoglobulin preparation (Swiss Red Cross Immunoglobulin, SRK-Ig) was tested both in vitro and in vivo. Using type III group B streptococci as a model system, the intravenous immunoglobulin preparation was shown to enhance phagocytosis and killing of the bacteria by human neutrophils. There was good activity to several type III strains, and for efficient opsonization both antibody and complement were required. In an animal model of neonatal group B streptococcal sepsis, passively administered immunoglobulin enhanced survival with 27/37 (73%) treated animals surviving compared to only 3/16 (19%) controls. These studies demonstrate that the intravenous immunoglobulin preparation SRK-Ig has retained functional activity and may have potential value for prophylaxis or therapy of certain bacteria infections.