We investigated the expression and the N-formyl-methionyl-phenylalanine (fMLP)- and interleukin-8 (IL-8)-induced mobilisation of the complement regulatory proteins CR1 (CD 35), decay-accelerating factor (CD55) and CD59 on neutrophils in neonates and adults. The expression of CD35 on resting neonatal neutrophils was significantly higher than in adults. Both fMLP and IL-8 increased CD35 expression more markedly in adults. CD55 on resting neutrophils in neonates was significantly higher than in adults, but did not further increase upon fMLP stimulation as opposed to adults. The increase in CD59 expression was more pronounced in adult neutrophils and was significantly higher than in neonates after fMLP activation. The addition of the protease inhibitor phenylmethanesulphonyl fluoride during fMLP activation improved the upregulation of CD 3 5 significantly more in neonates, but not CD 55 or CD59. This effect could be replaced by human normal sera. These data demonstrate that differences exist between neonatal and adult neutrophils with respect to the expression and mobilisation of functional receptors related to protection from autologous complement lysis and may indicate differences in the efficiency to circumvent complement-mediated cell damage at the inflammatory site.