Objective: To investigate the expression of CD40 and CD40 ligand (CD40L) on the surface of peripheral blood mononuclear cells(PBMCs) in asthmatic rats and the effect of anti-CD40L McAb on cytokines of PBMCs. Methods: Flow cytometry and RT-PCR were used to detect the expression of CD40 and CD40L of PBMCs in asthmatic rats. After the PBMCs was treated with anti-CD40L McAb, ELISA was used to detect the IL-4 and IFN-γ levels of culture supernatants. Results: Compared with the normal control group, the expression of CD40 and CD40L of PBMCs in asthmatic rats increased (P ＜ 0.05). Compared with the untreated group, the level of IL-4 and the ratio of IL4/IFN-γ decreased after the PBMCs was treated with anti-CD40L McAb(P ＜ 0.05). Conclusion: The expression of CD40 and CD40L on the surface of PBMCs in asthmatic rats was up-regulated.Anti-CD40L McAb can rectify the imbalance of Th1 and Th2 cytokines.