About 15–20% of bone marrow derived cells in the endometrium and the decidua are CD14-positive macrophages coexpressing the scavenger receptor CD163. The costimulatory molecule CD86 (B7-2) but not CD80 (B7-1) is found expressed on first trimester and term decidua macrophages (D-Mph). The number of D-Mph increases at the implantation site in the basal decidua, where they are in contact with the HLA-G-positive invading extravillous cytotrophoblast. As D-Mph express the inhibitory receptors CD85d (ILT4, LIR-2) and CD85e (ILT2, LIR-1), they are potentially susceptible to inhibition by class Ia and class Ib molecules. D-Mph may have specific functions relating to successful pregnancy, and phenotypic and functional properties may differ between early and term pregnancy. A number of data make their molecular repertoire comparable to those of alternatively activated macrophages which are immunosuppressive. For instance, first trimester decidua macrophages express IL-10 and the MS-1 high molecular weight antigen on the cell surface fibronectin and F XIIIa are detectable. Functional data are required to complement these studies.