The surface expression and regulation of the adhesion promoting glycoproteins Mac-1 and L-selectin was measured on monocytes and neutrophils from neonates and adults. A significant decrease in Mac-1 up regulation on both monocytes and neutrophils was found in neonates after both high (10(-7)M) and low (10(-9)M) concentrations of the chemotactic factor N-formyl-methionyl-phenylalanine (FMLP). A significant difference was obtained after incubation for five minutes, which was further enhanced after incubation for 15 minutes. Factors related to bacterial infections, lipopolysaccharides, activated sera (C5a), and aggregated IgG induced an impaired Mac-1 up regulation on both monocytes and neutrophils from neonates compared with adults. The expression of L-selectin was significantly lower on neutrophils from neonates and was less down regulated upon stimulation with a low concentration (10(-12)M) of FMLP. On monocytes from neonates, the expression and down regulation of L-selectin did not differ from monocytes from adults. Mode of delivery did not influence the regulation of Mac-1 and L-selectin in neonates. Diversity in expression and regulation of Mac-1 and L-selectin on monocytes and neutrophils may contribute to the increased susceptibility to infections observed in neonates.