Background: 17β-estradiol (E2) has been known to modulate immune response. Recent studies indicate that E2 at pregnancy level plays a role in regulating T cell response. Objective: To investigate the optimum dose of E2 (from 10-9 to 10-7 M) in mediating the generation of regulatory T cells (Tregs), using naïve human CD4+ T cells from healthy women. Methods: Naïve peripheral T cells were purified and conditioned with soluble anti-CD28 in anti-CD3-coated plates in the presence or absence of E2. Flow cytometry was employed to assess the expression pattern of forkhead boxP3 (FOXP3) and programmed death-1 (PD-1). Proliferation and cytokine secretions were analyzed, using XTT and ELISA assays. Results: In the presence of different doses of E2, the expression levels of anti-CD3/CD28 antibody-stimulated CD25/FOXP3 and FOXP3/PD-1 in conditioned T cells (cT) were peaked at 1 ng/ml (early pregnancy level, E2(1)) (47.14% (37.3-74.9) and 32% (27.7-52.5), respectively) and a slight, but not significant, increase after declining at 36 ng/ml (late pregnancy/pharmaceutical, E2(36)) (19.4% (15.2-24.5) and 15.8% (10.6-26.8), respectively). E2(1) cT showed a significantly reduced proliferation capacity (p<0.05) and secretion of IL-10 was enhanced in supernatants of E2(1 and 36) cT (p<0.05). In contrast to decreased TNF-a and IFN-g secretions in E2(1) cT supernatants, E2(36) stimulated TNF-a and IFN-g secretions (pConclusion: Our results indicate that the differential effect of E2 on generation of Tregs is consistent with the possibility that lower levels of pregnancy E2 are most efficient in induction of Tregs.