The role of human pregnancy-specific β1-glycoprotein (PSG) in the regulation of expression of the transcription factor FOXP3 was studied. It is found that, under conditions of directed induction of phenotypic changes in CD4+ lymphocytes to regulatory T cells (Treg), PSG at high concentrations (10 and 100 μg/mL) increased FOXP3 expression. The evaluation of the spontaneous expression level of FOXP3 mRNA showed that PSG (1 and 100 μg/mL) stimulated the expression of this factor in mononuclear cells and isolated CD4+ lymphocytes. Thus, PSG stimulates FOXP3 expression in immunocompetent cells.