G-CSF is a physiologic regulator of neutrophil production and function. Our group (Placenta 1996, Am J Obstet Gynecol 1996), and others, have shown that human placenta normally produces trace quantities of G-CSF in vitro and in vivo, but can generate large amounts during infection. However, the physiologic roles of placental G-CSF during normal gestation and intra-amniotic infection (IAI) are not clear. To provide clarification, we devised a series of experiments to test three hypotheses: 1) G-CSF in the maternal compartment of the placental circulation can cross to the fetal compartment, 2) transplacental passage of G-CSF involves cells at the maternal-fetal interface that produce G-CSF and other cells that express G-CSF receptors, and 3) G-CSF is detectable in the amniotic fluid (AF) and tracheal effluent throughout normal gestation, and is increased during IAI. To test the first hypothesis, 125I-G-CSF and unlabeled G-CSF were added to the maternal side of an in vitro perfusion system and G-CSF was measured (ELISA) in maternal and fetal perfusates. The second hypothesis was tested by evaluating placentas (5-40wks) for the presence of G-CSF and G-CSF-R by immunohistochemistry [n=10], RT-PCR [n=6], and Northern analysis [n=5]. To evaluate the third hypotheses, we measured G-CSF concentrations in AF(29-40wks gestation) [n=100] and in bronchoalveolar lavage (BAL) specimens[n=8] obtained on the first day of life. We conclude: 1) G-CSF can cross from the maternal to the fetal circulation (4-13% transfer rate; IgG=11% J Reprod Immunol 1983), 2) trophoblasts, decidua, and macrophages can synthesize G-CSF, 3) cells expressing G-CSF-R exist at the maternal-fetal interface and may be involved in this transfer, 4) G-CSF is present in AF throughout gestation in normal pregnancies (1100±199pg/mL, mean± SEM) and is elevated during IAI (10930± 2183pg/mL, p<.05), and 5) BAL from patients not exposed to IAI contained 473±155pg/mL G-CSF, while BAL of those exposed to IAI contained 15401±14099pg/mL G-CSF (p<.05). We speculate that the production and maternal to fetal transfer of G-CSF constitute important, previously undescribed, aspects of fetal host-defense during intra-amniotic infection.