Antibodies to differentiation markers have made it possible to identify macrophages in murine tissues. Macrophages are potent mediators of immunological reactions and it has been proposed that they are pivotal cells at the maternal-fetal interface. Studies of macrophage distribution in murine decidual tissue have provided conflicting evidence for and against the presence of significant numbers of macrophages at the maternal-fetal interface. The study described here used three independent macrophage differentiation antigens to examine the macrophage distribution in decidual tissue from day 6 to day 19 of pregnancy. Macrophage distribution was defined initially using a polyclonal antiserum to the plasma membrane differentiation antigen F4/80. Macrophages were virtually absent from antimesometrial decidual tissue until degenerating tissue was invaded by macrophages from about day 15. The resident population of macrophages in the mesometrial stroma was retained when this area decidualized but these cells did not survive beyond day 13. Mesometrial decidual tissue was virtually devoid of macrophages after this time. The metrial gland contained many macrophages until degeneration set in, but few were seen by day 15 of pregnancy. These distributions were confirmed using monoclonal antibodies (mAb) to F4/80, macrosialin, a monocyte- and macrophage-specific membrane sialoprotein (mAb FA/11), and the leucocyte beta 2-integrin CR3 (CD11b/CD18; Mac-1), which is expressed on neutrophils as well as monocytes and some tissue macrophages. CR3+, F4/80- and FA/11- neutrophils were found to be more widely distributed in decidual tissue than were F4/80+ or FA/11+ macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)