ABSTRACT: The leukocytic infiltrate in maternal uterine tissues at term in normal and preeclamptic pregnancies was studied immunohistologically using a number of monoclonal antibodies. No differences in the quantity and type of infiltrate are apparent between normal and preeclamptic pregnancy tissues. A large population of macrophages has been identified. Suppressor/cytotoxic T lymphocytes expressing major histocompatibility complex (MHC) class II products but lacking interleukin-2 receptor expression constitute the other component of the bone marrow-derived cell population. The suppressor/cytotoxic cells are seen around both arteries with and without physiologic changes and are seen also in acute atherosis. While it is suggested that the maternal macrophages serve to down-regulate maternal immune response, further functional studies of these leukocytes are required to define their role in normal pregnancy.