The depression in cell-mediated immune function following trauma-hemorrhage is shown to be restored by 17β-estradiol (E2) administration. However, it remains unknown which of the two estrogenreceptors, (ER)-α or ER-β, plays the predominant role in mediating the beneficial effects of E2. Female B57BL/J6 ER-β-/- transgenic mice [knockout (KO)] and corresponding ovariectomized wild-type (WT) mice were subjected to laparotomy and hemorrhagic shock (35.0±5.0 mmHg for 90 min) and treated with E2 (50 μg/25 g) or ER-α agonist propyl pyrazole triol (PPT; 50 μg/25 g) following trauma-hemorrhage. Four hours after resuscitation, systemic cytokine concentrations and cytokine release by splenocytes and splenic macrophages were determined by cytometric bead array. Trauma-hemorrhage resulted in a significant increase in plasma tumor necrosis factor α (TNF-α), interleukin (IL)-6, and IL-10. In contrast, the release of these cytokines by splenic macrophages was decreased significantly in WT and KO animals. Administration of E2 or PPT following trauma-hemorrhage produced a significant reduction in systemic TNF-α and IL-6 concentrations in WT and KO mice. Although the suppression in the productive capacity of these cytokines following trauma-hemorrhage by macrophages and splenocyte was also prevented in E2- and PPT-treated WT mice, the release of cytokines by macrophages and splenocytes in E2- and PPT-treated KO mice was not restored to the levels observed in sham animals. These findings collectively suggest that both receptors appear to play a significant role in mediating the immunoprotective effects of E2 in different tissue compartments following trauma-hemorrhage. © Society for Leukocyte Biology.