Objective To verify the presence of functional subsets of natural killer cells based on the cytokine production. Methods NK cells were purified and cultured in complete RPMI1640 medium in the presence of either IFNγ+anti-IL-4(classical Thl polarization) or IL-4 +anti-IFNγy(classical Th2 polarization) for three days, and then were collected and detected for type Ⅰ/type Ⅱ cytokines by RT-PCR method. Results NK cells were purified from 15 healthy donors, over 70% purity of NK cells were determined by flow cytometry. NK cells in peripheral blood expressed high level of type Ⅰ cytokines, mainly IFNγ, but low level of type Ⅱ cytokines such as IL-10 and IL-13, IL-4 was not produced by NK cells. Cells cultured in IFNγ+ anti-IL-4 condition exhibited significantly increased level of IFNγ, unchanged IL-2, and de creased type Ⅱ cytokines. Cells grew in IL-4 + anti-IFNγcondition exhibited increased IL-10 and IL-13,and decreased IFNγ expressions. Conclusions Based on the cytokine production, NK cells may be divided into two functional subsets in the same manner as that of T lymphocytes(e.g. Th1/Th2): .NKh1 and NKh2. The biological characterization and phenotypic marker are under investigate.