The human placenta is capable of producing a variety of hematopoietic growth factors in vitro. It is not clear, however, whether the placenta produces these factors in vivo, or if so whether such production has a physiologic role in fetal hematopoietic development. To assess this, we obtained human placentas between 10 weeks gestation and term and studied production of granulocyte colony-stimulating factor (G-CSF) in several ways. First, we sought to determine whether the onset of production of G-CSF mRNA in the placenta immediately precedes the appearance of neutrophil development in the fetus. From studying 26 abortuses, neutrophils first appeared at 14 weeks gestation. Placental G-CSF mRNA was not detected, however, in placentas of this gestation. In fact, placental G-CSF was not detected in any normal (non-infected) placentas from 10 weeks through term. We next assessed the effect of gestational age on the capacity of placentas to generate G-CSF, when stimulated in vitro by Interleukin-1 α (IL-1 α). Cubes of placenta, obtained at various gestations, were incubated overnight in tissue culture media. Relatively small quantities of G-CSF were liberated into the culture media (226±14 pg G-CSF/mL, mean±SD) from placentas of 17-34 weeks gestation (n=6), and this did not increase when IL-1 α was included (232±15 pg/mL). Placenta from term gestations (n=5), however liberated 1,168±419 pg G-CSF/mL into the culture media to which IL-1 α was not included, and 24,878±6319 in those in which it was not. Next, we assessed the rate of G-CSF production by placentas, by perfusing two normal, term placental cotyledons, using a membrane-oxygenator system, and quantifying G-CSF, at intervals, in the perfusate. In both, production of G-CSF peaked at >25 pg G-CSF/mL placental tissue/hr, and continued for 48 hrs. Thus, 1) we found no evidence that placental production of G-CSF is involved in regulating fetal granulocytopoiesis, 2) healthy placentas produce little or no G-CSF in vivo, 3) the placenta at term has a far greater capacity to produce G-CSF, when stimulated, than does the placenta before term, and 4), although the placenta at term does not normally produce G-CSF, it has the capacity of generating very large quantities of G-CSF continuously over at least two days.