In previous studies, we have reported that after chemotactic factor stimulation, PMNs from neonates fail to undergo certain critical activation steps. Furthermore, the concentration of free intracellular calcium reached is significantly below that of PMNs from adults. Interferon-gamma (IFN-gamma) is a lymphokine that has been shown to activate phagocytic cells, and IFN-gamma messenger RNA production by neonatal mononuclear leukocytes has been reported to be depressed. In the present studies, we found that recombinant human IFN-gamma markedly enhanced the chemotactic responses of PMNs from neonates to levels that were not different from that of PMNs from adults. Furthermore, preincubation of the neonatal cells with this recombinant human lymphokine also corrected the abnormality in intracellular calcium metabolism. These results suggest that this developmental defect in phagocytic cell movement may be the result of an intrinsic defect in IFN-gamma production resulting in deficiency of this critical phagocyte-activating lymphokine.