Little is known about phagocytosis-associated oxidative metabolism in mononuclear phagocytes from the human neonate. We investigated this phenomenon in monocytes from the cord blood of term newborn infants by measuring generation of superoxide anion (O2-) and hydroxyl radical (X OH) after stimulation with opsonized zymosan or phorbol myristate acetate. Production of these microbicidal oxygen metabolites by monocytes from neonates and healthy adult volunteers was equivalent. When cultured in the presence of the macrophage activator lipopolysaccharide or muramyl dipeptide, monocytes from neonates and adults differentiated into cells with the appearance of macrophages and with an enhanced capacity to release O2- compared with cells cultured in the absence of an activator. Monocyte-derived macrophages from neonates produced only slightly less O2- than did adult cells. Thus, unlike the cord blood neutrophil, which exhibits abnormalities in oxidative metabolism, the cord blood mononuclear phagocyte has a respiratory burst that is quantitatively comparable to that of the adult cell.