Decreased adhesion of neutrophils to endothelial cells and delayed transendothelial cell migration of neutrophils have been consistently reported in neonatal animals and humans and contribute to their susceptibility to infection. The delayed transmigration of neutrophils is especially prevalent in premature neonates. To define the nature of this defect, we used an in vivo animal model of inflammation and found that radiolabeled leukocytes from adult rats transmigrated into the peritoneum of other adult rats 5 times more efficiently than they did in neonatal rats (P =.05). This indicated that defects in neonatal neutrophils could not completely account for the delayed transmigration. Delayed transmigration in the neonatal rats correlated with a defect in the expression of P-selectin on the surface of their endothelial cells. We found a similar P-selectin deficiency in endothelial cells lining mesenteric venules and umbilical veins of human premature infants when compared with term human infants. The decreased P-selectin in premature infants was associated with decreased numbers of P-selectin storage granules and decreased P-selectin transcription. Decreased P-selectin expression on the surface of endothelial cells in preterm infants may contribute to delayed neutrophil transmigration and increased susceptibility to infection.