Changes in the expression of antigens on mouse uterine or embryonic tissues were examined by enzyme immunocytochemistry. Cryostat sections of uteri from days 1, 8 and 15 of pregnancy were probed with the monoclonal antibodies M3/38 and M3/84, originally defined by their reactivity with macrophage surface antigens (Mac-2 and Mac-3, respectively). In the uterus of pregnant mice, reaction of these antibodies was not limited to leucocytes. M3/38 did not react at detectable levels with cells in the uterus on day 1 but did react with decidual cells immediately surrounding the embryo on day 8. By day 15, the placenta, decidua basalis and metrial gland were intensely positive but the embryo was negative. M3/84 reacted with the luminal side of the endometrium on day 1, the entire decidual mass on day 8, and with all maternal and fetal tissues on day 15. M3/38 was detected in saline-soluble preparations of uterine tissue and had a molecular mass of approximately 32-35 kDa as determined by SDS-PAGE and western blot analysis. The pattern of expression of these molecules suggests a functional relationship to developmental changes that occur at the maternal-fetal interface.