Sera were obtained from 52 mothers and their infants, from 36 additional children from 1 week to 3 years of age, from 15 individuals with congenital or acquired agammaglobulinemia, and from 2 children with type 1 dysgammaglobulinemia. The bactericidal activity of these sera against Salmonella typhosa 0901 and Escherichia coli 0127 in the presence of human complement was studied. Selected maternal and cord sera were fractionated by electrophoresis and by ultracentrifugation, and the bactericidal activity of the fractions also studied. The following results were obtained. Most of the newborns lacked serum bactericidins against 0901 and 0127, and in those infants where bactericidins were found, the activity was low compared to that of the mother. When there were no detectable serum bactericidins in the newborn infant, all or almost all of the activity in the mother's serum was present as 195 gamma1-globulin. When the newborn infant did possess serum bactericidins, activity similar in amount to that in the infant's serum was present in the mother's 7S gamma2-globulins, although most of the activity of the maternal serum was 19S gamma1-globulin. Bactericidins in cord sera, when present, separated with the lower molecular weight serum proteins, and none was found in the 195 macroglobulin fraction. There was little or no bactericidal activity in sera from patients with agammaglobulinemia, and high concentrations in two children with type 1 dysgammaglobulinemia. The bactericidins measured were specific for each of the two organisms tested and appear to be specific antibodies. The data are in accord with the observation that the newborn's serum is markedly deficient in 19S gamma1-globulins and that there is transplacental transfer of 7S gamma2-globulins from mother to fetus. As a result the newborn infant is deficient in bactericidins to Gram-negative enteric bacilli.