Intercellular adhesion molecule 1 (ICAM-1), a ligand of leukocyte function associated antigen 1 (LFA-1), is present on many cells, including monocyte/macrophages. ICAM-1 is considered to play an important role in the induction and maintenance of inflammatory responses by permitting leukocyte adhesion. Its expression is inducible on endothelial and epithelial cells exposed to various inflammatory cytokines (preceding expression of HLA-DR) and is maturation dependent in certain cell lines. The distribution of ICAM-1 in decidua and placenta was evaluated using peroxidase-antiperoxidase immunohistochemistry. In decidua of first and third trimesters, scattered ICAM-1-staining cells were observed. In placentas of first and third trimesters, all types of trophoblasts stained negatively for ICAM-1. Prior to 10 weeks of gestation, the villous stroma was uniformly ICAM-1 and HLA-DR unreactive. Beginning in the chorionic plate at approximately 10 weeks, scattered ICAM-1-positive stromal cells were observed, whereas stromal cells of the terminal villi revealed no ICAM-1. By 14-16 weeks, approximately 40-50% of the terminal villous stromal cells were ICAM-1 staining. This parallels the 40-50% of the villous stromal cells that share other immunohistochemical markers, such as EB-11, with monocyte/macrophages. The lack of functional maturation of the villous stromal macrophage may explain the rarity of chronic villitis early in gestation.