Uterine dendritic cells (DCs) are critical for activating the T cell response mediating maternal immune tolerance of the semiallogeneicfetus. GM-CSF (CSF2), a known regulator of DCs, is synthesized by uterine epithelial cells during induction of tolerance in early pregnancy. To investigate the role of GM-CSF in regulating uterine DCs and macrophages, Csf2-null mutant and wild-type mice were evaluated at estrus, and in the periconceptual and peri-implantation periods. Immunohistochemistry showed no effect of GM-CSF deficiency on numbers of uterine CD11c+ cells and F4/80+ macrophages at estrus or on days 0.5 and 3.5 postcoitum, but MHC class II+ and class A scavenger receptor+ cells were fewer. Flow cytometry revealed reduced CD80 and CD86 expression by uterine CD11c+ cells and reduced MHC class II in both CD11c+ and F4/80+ cells from GM-CSF–deficient mice. CD80 and CD86 were induced in Csf2−/− uterine CD11c+ cells by culture with GM-CSF. Substantially reduced ability to activate both CD4+ and CD8+ T cells in vivo was evident after delivery of OVA Ag by mating with Act-mOVA males or transcervical administration of OVA peptides. This study shows that GM-CSF regulates the efficiency with which uterine DCs and macrophages activate T cells, and it is essential for optimal MHC class II-and class I-mediated indirect presentation of reproductive Ags. Insufficient GM-CSF may impair generation of T cell-mediated immune tolerance at the outset of pregnancy and may contribute to the altered DC profile and dysregulated T cell tolerance evident in infertility, miscarriage, and preeclampsia.