The experiment was performed to investigate changes of maternal immune status during the pregnancy. We observed the distribution of several immune cells [macrophage, activated B-cell, IgM+ B-cell, Lyt-2+ T-cell and L3T4+ T-cells] in the utero-placental interface and the peripheral blood of Balb/c mice. The experimental animals were divided into seven groups by the gestational ages ; virgin, 2nd, 5th, 8th, 10th, 14th and 19th day of pregnancy. In the utero-placental interface, the distribution patterns of the lymphocytes [both T and B] and macrophages were observed. Histochemical staining by naphthol-AS-MX phosphate sodium salt was used for the detection of activated B-cells. For the detection of macrophage, plasma cell, suppressor cell and helper cell, all the prepared samples reacted with Rat anti-mouse Mac-1, goat anti-mouse IgM, rat anti-Lyt2 and rat anti-L3T4 antibody first, and washed. Second reaction was done with biotinylated anti-rat or anti-mouse IgG anti-bodies, and washed. Avidin-biotin -peroxidase complex and 3, 3`-diamino-benzidine[DAB] were used for the visualization of specific cells. T-cells and B-cells were not observed during the all stages of pregnancy. By contrast, macrophages were observed a few at the perimetrium on the second day of gestation, and they were found at the outermost portion of the trophoblastic layer on the 8th day, and they were observed at the decidua basalis in late pregnancy after the 10th day when the placenta were well developed. In the peripheral blood, activated B-cells were not observed throughout the pregnancy. On the 8th day, the proportion of plasma cells to total mononuclear cells was decreased significantly to 16+/-2.4% compared with the virgin group[22+/-2.6%][p<0.01]. It increased again and it reached 42+/-5.8% on the 14th day and 37+/-4.9% on the 19th day. Helper T-cells were decreased on the 14th day[30+/-2.4%] compared with the normal control[47+/-5.1%]. But, Suppressor T-cells were increased on the 8th day[35+/-2.9%] and the 10th day[33+/-3.6%] compared with the normal controls[27+/-2.3%]. This decrement returned to the level of the normal controls on the 14th day and 19th day. Together with our previous data, we could find the synchronized changes of immune cells in utero-placental interface, uterus draining lymph nodes, peripheral blood and spleen. Therefore, we suggest that macrophages in utero-placental interface may play an important role for the immune responses against the fetal transplantation antigen.