Macrophages are abundant in the mesenchymal and connective tissue stroma of the cycling and pregnant uterus, and constitute a significant proportion of the villous or labyrinthine mesenchymal cells in the human and murid placenta. In other contexts, the activities of these multifunctional cells are strongly influenced by regulatory molecules such as steroid hormones, polypeptide growth factors, and bioactive lipids. All of these are present at particularly high concentrations in the pregnant uterus and placenta. Thus, uterine and placental macrophages stimulated by endogenous factors could contribute to the complex cellular and molecular interactions that result in successful pregnancy.