Objective: To study the effects of Qiling capsule(QLC) on natural killer cell (NK) activity in mice. Methods:The cytotoxicity was detected by 3H-TdR release assay and levels of interferon (IFN) in serum by cellular protected method. Results: QLC had the ability to improve the cytotoxicity which was either normal or inhibited by cyclophosphamide(CY)(P<0.05) in vitro. Moreover,in vivo,QLC could increase the NK activity (P<0.05) and raise the serum levels of IFN(P<0.01)in mice pretreated by CY,but no effect on normal mice. Conclusion: The results inferred that the QLC protection of NK against CY inhibition might be related to enhancement of IFN levels.