Abstract： Objective To investigate the effeet of eyclosporine A (CsA) on CD4+ Foxp3+ T cells in C57BL/6 mice and the underlying mechanism.Methods C57BL/6 mice, aging 8～ 10 weeks, were administered intraperitoneally with CsA at a dose of 20 mg· kg-1·d-1 for 2 weeks, while the control mice given sterile PBS.After 2 weeks, peripheral blood was collected, and splenoeytes and thymoeytes were prepared for the detection of CD4 + Foxp3 + T cells by using flow cytometry.Results The proportion of CD4+ Foxp3+ T cells in peripheral blood, spleen and thymus of CsA-treated mice was (0.581 ± 0.089) %, (2.189 ± 0.046) % and (0.472 ± 0.049) %, while that in PBS-treated mice was (1.751 ± 0.227) %, (3.684 ± 0.169) % and (1.412 ± 0.188) %, respectively (P < 0.001).Conclusion The reduction of CD4 + Foxp3+ T cells in CsA-treated mice suggests that CsA can inhibit the development of regulatory T cells.