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 immune response, further functional studies of these leukocytes are required to define their role in normal pregnancy.