In blood vessels of the chorionic villi of mouse placenta, primitive macrophages first emerged at 10 days of gestation, then differentiated and matured into fetal macrophages. After emigration into the chorionic villous mesenchymal stroma, they ingested fluid-like stromal materials and transformed into Hofbauer cells. In our observation of their differentiation and maturation, no promonocytes or monocytes were detected. In the culture of cell suspensions from the placenta with LP3-conditioned medium, CFU-GM was confirmed, but in the culture with the mouse bone marrow stromal cell line (ST2) the primitive/fetal macrophage population occurred predominantly before the development of the monocyte/macrophage population. Proliferative potential of the primitive and fetal macrophages is slight. With the progress of gestation, the monocyte/macrophage population appeared in the chorionic villous stroma, forming a heterogeneous population of placental macrophages in the late fetal stage.