The immunosuppressive drugs Cyclosporine A (CSA) and prednisolone are widely used to prevent graftversus- host-disease (GVHD) after allogenetic stem cell transplantation (allo-SCT). However, it remains to be elucidated whether these drugs have an effect on the balance of effector cells and CD4+CD25+ regulatory T cells (Tregs) which determines the development of stable allograft tolerance. In the current study, we found that proliferation, activation and function of T cells especially for virus-specific CD8+ T cells and Tregs were inhibited by CSA and prednisolone in a dosedependent manner. These effects were associated with lower secretion of cytokines and arresting T cells in the G0/G1 phase of cell cycle. Moreover, CSA and prednisolone could reduce the expression of FOXP3 and suppressive function of Tregs. Our data indicated that CSA and prednisolone impaired the development and function of CD8+ T cells as well as Tregs in a similar way suggesting that CSA and prednisolone might increase the susceptibility to develop viral diseases and block the potential induction of immune tolerance in clinical settings.