Maturational and pathologic alterations in the phagocytic, opsonic, and antibody system of term and low birth weight infants will be delineated, and their relative contribution to the newborn's capacity to ingest and kill bacteria assessed. The over-all objective is to define immunologic deficiencies of newborns, especially of prematures, which may in turn lead to rational preventive or corrective measures. Previously described deficiencies of each system will be reevaluated and extended by newer, more precise methods; these shall include quantitative iodination to measure killing capacity, facilitation of iodination in the presence of different particles or bacteria, to measure opsonic function, and measurement of heat stabile opsonins, bacterial agglutinins, and IgG subclasses to define antibody function. The phagocytic and killing ability of leukocytes of high risk infants with various neonatal disorders such as hyperbilirubinemia, hypoglycemia, respiratory distress syndrome, etc. will be defined; this will provide information as to how these disorders impair neonatal resistance to infection. Comparison of the phagocytic, opsonic and antibody systems of term and low birth weight infants of various gestational ages will indicate the maturational timetable of these functions and indicate their interdependence. Therapeutic measures to correct phagocytic, opsonic and antibody deficiencies such as plasma, blood, or gamma globulin will be assessed.