An immunohistochemical study of stromal cells in human placental villi was made using various polyclonal and monoclonal antibodies. This study demonstrated that almost all villous stromal cells expressed HLA-ABC, which is indicative of class I major histocompatibility complex, and vimentin, which is a mesenchymal marker, through the entire period of pregnancy. Some of these stromal cells were considered to be fetal macrophages having HLA-DR, which is a determinant of class II major histocompatibility complex, particularly in the second and third trimesters. Such macrophages also expressed CD-4 (Leu-3a and -3b) and 2H4, which are the cell membrane determinants of suppressor-inducer T lymphocyte; CD-2 (Leu-5b), which is a marker of pan-T cell; Leu-M3, Leu-M5 and Mac-1, which are the markers of monocyte and macrophage lineage; leukocyte common antigen, which is a marker of bone marrow derived cell; and α-antichymotrypsin, which is a glycoprotein associated with macrophages. Morphologically, villous macrophages consisted of heterogeneous phenotypes such as classic Hofbauer cells, fibroblast-like spindle cells with long cytoplasmic processes, and dendritic-shaped cells. These may have more complex features than previously considered and may have a greater initiating role in immunologic interactions between mother and fetus when compared with the very sparsely distributed T or B lymphocytes.