Bacteria such as Escherichia coli and group B streptococci are common neonatal pathogens. Neonates infected perinatally often develop overwhelming sepsis which may rapidly progress to shock and death in just a few hours. This fulminant course suggests that immunity to these bacteria is deficient. Studies from several laboratories have shown that antibody is important in immunity to group B streptococcus. In vitro studies have demonstrated that efficient phagocytosis and killing of these bacteria by neutrophils or monocytes requires opsonic antibody. Many premature babies have decreased amounts of total serum immunoglobulin G and term babies may not have sufficient levels of group B streptococcal antibodies to be protected. Immune globulin that contains adequate opsonic antibody to group B streptococcus may therefore be of value as adjunctive therapy for treating infections with these bacteria and may reduce the morbidity and mortality associated with group B streptococcal infection in high-risk neonates. Although intravenous immune globulin has been used to both prevent and treat infections in neonates, only limited efficacy data are available. Several large, blinded, and controlled studies that are being completed and analyzed will be important to determine the role of immune globulin therapy in neonates.