Neonatal host defense simulates a clinical state of immunodeficiency that predisposes the preterm and term newborn to overwhelming bacterial sepsis. Defects in both the quantitative and the qualitative aspects of the neonatal phagocyte contribute significantly to the immaturity of their immune system. The neonate lacks adequate numbers of granulocyte bone marrow progenitor cells and has a decreased neutrophil storage pool and an increased tendency to peripheral neutropenia during neonatal sepsis. Additionally, the neonatal granulocyte demonstrates altered physiological function compared with that found in the adult with respect to chemotaxis, phagocytosis, oxidative metabolism, and bacterial killing. Reduced circulating immunoglobulins and impaired production of specific antibody are additional hallmarks of altered neonatal immunity. The use of adult neutrophil transfusions for the treatment of neonatal sepsis has shown promise in some clinical studies and no difference in others. Recent investigations have examined the use of intravenous gamma-globulin for prophylaxis and treatment of neonatal sepsis. The following review summarizes the state of immunodeficiency in the newborn and the potential role of polymorphonuclear leukocyte transfusions in the treatment of overwhelming neonatal bacterial sepsis.