We describe a simple test for the evaluation of phagocytosis and provide a chart of reference values to evaluate normal phagocytosis by age. We assessed the postnatal maturation of phagocytic function of neutrophils and monocytes. Phagocytosis was evaluated in newborn children delivered vaginally or by cesarean section, infants, preschool children, schoolchildren, and adult subjects. Two drops of blood were placed on a microscope slide and incubated with Saccharomyces cerevisiae yeasts, and phagocytosis was evaluated by microscopy. Our technique showed results comparable to or better than those obtained by other usual techniques. The neutrophils of newborn children delivered by cesarean section showed a phagocytic capacity 45% higher than those of neonates delivered vaginally, whereas neutrophils from children in the latter group showed the lowest phagocytic capacity of all age groups. Phagocytosis by neutrophils reached the levels seen in adults at about the first year of life, while there were no important variations in phagocytosis by monocytes in the different age groups. The technique described is reliable and fast, uses only a few drops of blood, and allows better preservation of cell function due to the minimal manipulation to which the cells are submitted. The delayed maturation of the phagocytic function by neutrophils may account for the high levels of susceptibility of newborn and infant children to bacterial infections. This practical method of assessment of phagocytosis may allow the diagnosis of primary or secondary phagocytic deficiencies to be made more easily and may allow better monitoring and treatment of those with dysfunctions of these cells.Extracellular bacteria are the main agents of infections in neonates and infant children, and these pathogens depend on phagocytes for their elimination. Gram-negative enteric rods, Staphylococcus aureus, and Streptococcus spp. are the major causal agents of infections in newborn children, whereas Haemophilus influenzae and Streptococcus pneumoniae predominate in young infants (24,27,36). Antibodies and complement factors may act together with phagocytes in the defense against these infectious agents (4). Indeed, it has been shown that deficiencies in phagocytosis, humoral immunity, and complement factors during the first months of life increase susceptibility to infectious diseases and cause high rates of mortality in this period of life (4, 29).The evaluation of phagocytic function in children has shown conflicting results (4, 16). Some investigators have found phagocytosis to be normal, whereas others have shown deficiencies. It has been observed that phagocytosis by neonatal leukocytes is abnormal when these cells are suspended in neonatal serum but not in adult serum, suggesting a role for a deficiency of opsonin components in the serum of newborn children. However, the phagocytic function in healthy children is not completely clarified (4, 16).Although testing of phagocytic function has long been used for evaluation of the defense mechanisms of chi...