Although there are many studies on the effect of granulocyte and granulocyte-macrophage colony stimulating factors (G/GM-CSF) on adult neutrophil functions, there is little information regarding their influence on neonatal cells. We studied the in vitro effect of G/GM-CSF on neutrophil chemotaxis, polarization, and superoxide anion generation in 47 neonates compared with 35 adults. We found that G-CSF and GM-CSF significantly enhanced the chemotaxis of newborn infants' neutrophils, normalizing their chemotactic defect [from 35 +/- 7 cells/field (mean +/- SE) to 49 +/- 5 cells/field with G-CSF, p < 0.05 and to 55 +/- 4 cells/field with GM-CSF, p < 0.001]. It is notable that the maximal neutrophil response to the cytokines was observed particularly in the newborn infants with severe impairment in their chemotactic activity. Statistical analysis of the data showed a significant inverse correlation, which supported this observation (r = -0.6, p < 0.02 for G-CSF; r = -0.76, p < 0.001 for GM-CSF). The reduced polarization of neonatal compared with adult cells [71 +/- 5% versus 86 +/- 2% (mean +/- SE), p < 0.05], was corrected by CSF-priming (to 87 +/- 4% with G-CSF and to 92 +/- 2% with GM-CSF, p < 0.05). In addition, the neutrophil superoxide generation was significantly improved in both groups following the CSF-priming. GM-CSF and G-CSF gave comparable results in all functions studied except that GM-CSF improved superoxide release to a greater extent. This study shows a significant improvement of the neonatal neutrophil functions following in vitro CSF-priming and contributes to a better understanding of the neonatal neutrophil behavior when treated with G/GM-CSF.