Objective:To estimate the possible effect of placental monocyte-macrophages on the pregnant immunological tolerance by observing the expression of indoleamine 2,3-dioxygenase(IDO) and the secretion of IL-10 and IL-12 in cells from different gestational phages. Methods:Monocyte-macrophages cells were separated mechanically from normal term (39 weeks ± 5 days,25 cases) and midtrimester (21 weeks ± 3 days,19 cases) placental tissues. The cytokines including IL-12 and IL-10 were assayed by ELISA,while the expression of IDO was measured by RT-PCR and Western blot,respectively. The monocyte-macrophages cells derived from placental tissue in different gestational phages were co-cultured with allogeneic T cells in order to detect their abilities in stimulating the proliferation of allogeneic T cells detected by CCK-8. Results:Monocyte-macrophages cells from normal term placental tissues secreted high level of IL-12,but expressed low levels of IL-10 and IDO. Besides,monocyte-macrophages cells from normal term placental tissues were more powerful in promoting the proliferation of allogeneic T cells than the cells from midtrimester term placental tissues. Conclusion:Monocyte-macrophages cells from normal term and midtrimester placental tissues may play roles in the maintenance of pregnant immunological tolerance and delivery possibly by placental tissue in different gestational phages possess different biological characters,which possibly by regulating the IL-12/IL-10 balance and IDO expression. However,the association of IL-12/IL-10 balance with the expression of IDO needs further elucidation.