We have studied the role of pregnancy hormone (chorionic gonadotropin, CG) in the control of expression of Foxp3 (a marker of T-regulatory lymphocytes, Treg), synthesis of interleukin-4 (IL-4) and interferon-gamma (IFH-γ) in lymphocytes, and secretion of indolamine-2,3-dioxygenase (IDO) by monocytes. Moreover, we evaluated the phagocytic and oxidative activities of these cells. It was shown that incubation of CG at a dose of 100 IU/ml with mononuclear cells from peripheral blood (MPC) resulted in a rise in the relative number of IL-4+CD3+CD4+T-lymphocytes whereas the number of IFN-γ+CD3+CD4+T-cells remained unaltered; the intracellular production of IFN-γ and IL-4 in the subpopulation of CD3+CD8+T-lymphocytes did not change either. In vitro, CG (10 and 100 IU/ml) significantly increased percentage of CD4+CD25 brightFoxp3+ cells and enhanced activity of indolamine-2,3-doxygenase in lipopolysaccharide-stimulated MPC. CG suppressed phagocitosis of E. coli lux+ by monocytes and simultaneously inhibited the production of active oxygen species in the reaction of spontaneous luminol-dependent chemiluminescence. Taken together, these properties of CG account for its role in the promotion of the formation of immunological tolerance during pregnancy.