We analyzed the production and expression of three colony-stimulating factors (CSFs) in neonates to clarify the mechanism of leukocytosis at birth. Serial blood samples (n = 23) were collected from mothers, cord blood, and from newborn infants on days 1,5, and 30 after birth. The serum levels of granulo-cyte-CSF (G-CSF), granulocyte/macrophage-CSF (GM-CSF) and macro-phage-CSF (M-CSF) were measured by ELISA. The G-CSF levels on day 1 after birth were significantly higher than those thereafter, and they were also higher in the mothers than those on days 5 and 30 after birth. The GM-CSF levels did not change significantly during the neonatal period. The serum M-CSF levels were higher on postnatal day 1 than at other times, and gradually decreased thereafter. To confirm the production sites of G-CSF and M-CSF, the mRNA for these CSFs in peripheral mononuclear cells (MNCs) from healthy adults, mothers, and cord blood were analyzed by PCR. The expression of G-CSF and GM-CSF mRNA was undetectable in MNCs from adults, mothers, and cord blood, while these cells expressed low levels of M-CSF mRNA. After stimulation with lipopolysaccharide or phorbol myristate acetate, the MNCs expressed high levels of G-CSF and GM-CSF mRNA. The levels of G-CSF PCR products in cord MNCs were lower than those in adult and maternal MNCs. The expression of M-CSF mRNA was virtually unchanged by stimulation. To detect the localization of G-CSF and M-CSF in the placenta and umbilical cord, these tissues were immunocytochemically stained with anti-G-CSF and anti-M-CSF antibodies. G-CSF and M-CSF were expressed in trophoblasts and decidual stromal cells, whereas the umbilical cord did not express these CSFs. Moreover, large amounts of G-CSF and M-CSF were detected in the supernatant of cultured trophoblasts and decidual stromal cells. The expression of G-CSF and M-CSF in these cells was confirmed by PCR. These findings suggested that G-CSF and M-CSF produced in the placenta (trophoblasts and decidual stromal cells) are the major factors that induce leukocytosis in newborn infants at birth.