AbstractGranulocyte-macrophage colony-stimulating factor (GM-CSF) is produced by cells in the placenta, is known to be a growth factor for trophoblast cells in vitro and when injected into pregnant mice at risk for mid-gestation fetal resorption, dramatically lowers the fetal death rate while stimulating placental and fetal growth. We describe here the localization of GM-CSF mRNA expression in murine placenta by in situ hybridization. It is found in small round cells (lymphoid-like) and endothelial cells in the maternal decidua. In addition, GM-CSF transcripts are located in cells of the spongiotrophoblast zone (trophoblast-like cells), but not in the labyrinthine zone. These results indicate that GM-CSF may be influencing the growth and function of the fetal placenta in a paracrine-autocrine manner. These results support earlier observations that link GM-CSF production during pregnancy to decidual T-lymphocytes and further suggest a placental source within the invasive trophoblast.