ABSTRACT.Cord blood phagocytic cells were characterized with respect to cytochemical activities, FCγ and C3b receptors, and capacity to phagocytose and kill various species of bacteria.The percentages of peroxidase-positive granulocytes and monocytes from neonates and adults were comparable; the percentage of esterase-positive cord-blood monocytes was about two-thirds of that of adults' blood monocytes. The numbers of cord blood and adults' monocytes with FCγ and C3b receptors were similar.Phagocytic and intracellular killing capacities of cord blood granulocytes and monocytes were investigated forStaphylococcus aureus, Escherichia coli, Streptococcus pyogenes,and group BStreptococcustype III. The rates of phagocytosis and intracellular killing by granulocytes from newborns proved to be comparable to the corresponding values for granulocytes from adults.Cord blood monocytes phagocytosedE. coli, S. aureus,andS. pyogenesat a normal rate, and killed ingestedE. coliandS. pyogenesintracellular at the same rate as adults' monocytes did. However,S. aureuswas killed at a much lower rate by cord blood monocytes than by monocytes from adults. Phagocytic activity for group BStreptococcuswas impaired and killing of these bacteria by cord blood monocytes was virtually nil. The latter finding might partially explain the frequent streptococcal infections in newborns.