Because increased complement receptor expression is necessary for optimal function of adult neutrophils, we tested the hypothesis that the increased susceptibility of neonates to infection might be due to an impaired ability of neonatal neutrophils to increase expression of complement receptors in response to chemotactic stimuli. We used monoclonal antibodies and flow cytometry to compare surface expression of the receptors for the complement components C3b (CR1) and C3bi (CR3) on adult and neonatal cord blood neutrophils (PMNs). We also compared receptor expression on PMNs from infants delivered by cesarean section without labor versus infants delivered vaginally. Expression of both CR1 and CR3 was minimal on resting adult and neonatal PMNs maintained at 0 degrees C. There was a modest increase in expression of both receptors when PMNs were warmed to 37 degrees C. This increase was similar on adult and neonatal cells, both unfractionated in whole blood and after isolation with Percoll density centrifugation, with one exception. Expression of CR1 was greater on isolated PMNs from vaginally delivered infants versus adults when the cells were warmed to 37 degrees C. This difference was not observed with cells from infants delivered by cesarean section without labor, suggesting this modest increase in receptor expression may be due to factors associated with labor. When isolated cells were stimulated with either N-formyl-methionyl-leucyl-phenylalanine or zymosan-activated serum, expression of CR1 increased to the same extent in both neonatal and adult PMNs. In contrast, maximal CR3 expression on cord PMNs stimulated with N-formyl-methionyl-leucyl-phenylalanine or zymosan-activated serum was only 75% of the adult values.(ABSTRACT TRUNCATED AT 250 WORDS)