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Cordyceps sinensis (Cs) or Dongchongxiacao in Chinese is a rare and precious medicinal mushroom. In this work,a Tolypocladium sp. fungus named Cs-HK1 was isolated from a natural Cs fruiting body,and its ultraviolet (UV),infrared (IR) and high-performance liquid chromatography(HPLC) spectra all exhibited high degrees of matching to those of natural Cs,indicating the similarity of chemical composition. A crude polysac-charide (PS) fraction isolated from Cs-HK1 fungal mycelium showed significant antitumor activity,inhibiting melanoma tumor growth in mice. The PS-treated mice showed significantly higher serum interferon-γ(IFN-γ) level and higher proliferation rate of lymphocytes ex vivo than the control animals,suggestive of the immuno-modulatory activity of the PS fraction. In cell cultures,the PS fraction caused a significant and dose-dependent inhibitory effect on the proliferation of two cancer cell lines ,B16/neu mouse melanoma and MDA-MB-231 human breast cancer cells,but not on normal spleen lymphocytes in vitro. Most of these bioactivities of the Cs-HK1 fungus were comparable to or even greater than those of natural Cs. The results showed that Cs-HK1 fun-gus may be a functional substitute for the natural Cs in health food and herbal medicine.