The newborn infant is particularly susceptible to infection in the first weeks of life and this may be, in part, related to functional impairment of neonatal neutrophils in regard to adherence, chemotaxis, and migration. Differences in expression of the neutrophil adherence molecules, L-selectin and CD11b/CD18 (Mac-1), have been previously demonstrated in cord blood and in very young infants (</= 48 h) compared with adults, but it is unknown for how long these differences persist. We measured surface expression of neutrophil L-selectin and CD11b using flow cytometry in healthy term human newborns from 24 h through 4 wk of age. We also measured levels of soluble L-selectin using an ELISA in neonates though the age of 4 wk. Compared with adults, neonates expressed significantly less L-selectin on resting neutrophils through 4 wk of age, with the lowest levels being at 24 h. Expression of L-selectin on the neutrophil after activation with N-formyl-methionyl-leucyl-phenylalanine was less in the neonate than in the adult (p < 0.05) for the first week of life. Soluble L-selectin showed a steady increase with age in the neonates. Soluble L-selectin was significantly lower in the 24-h neonate compared with the adult and was higher in the 4-wk neonate compared with the adult. CD11b expression was similar between neonates and adults on unstimulated neutrophils, but upon activation, the neonatal neutrophil demonstrated significantly lower up-regulation of CD11b on the neutrophil surface through 4 wk of age compared with adults (p < 0.05 for all ages except 1 wk). These findings that differences in expression of L-selectin and CD11b from that in adults persist throughout the neonatal period provide further evidence that these differences may play a role in the neutrophil defects observed during the neonatal period.