：Objective To studywhether the primary ovarian cancer cells containing cancer stem cells and itscharacterization in serum-free culture condition.Methods The primary cancer cells wereisolated from one stage Ⅲ, grade 2 serous adenocareinoma tissue.Cells were culturedin serum-free culture system supplemented with epidermal growth factor,basic fibroblastgrowth factor,leukemia inhibitory factor and insulin.or standard serum-containingsystem.Methyl thiazolyl tetrazolium assay,quantitative PCR analysis,flow-cytometricanalysis and xenograft experiments in vivo were performed.Results The primary cancer cellscould maintain and forill cell sphere in serum-free culture system.These cells had theproperties of self-renewal,overexpression of stem cell marker genesNanog,Oct-4,Sox-2,nestin,ABCG2,CD_(133) and CD_(117) By contrast with the difierentiatedcells under standard serum-containing culture conditions.these sphere-forming cells weremore resistant to cisplatin and paclitaxel after treated 48 and 72 hours(61% vs.31%,73%vs.29%,P<0.05).With Hoechst 33342 exclusion assay,only 21.83%of sphere-forming cellswere positive with the dye,compared with 83.04% positive cells in differentiated cells(P<0.01).Only 500 sphere-forming cells resulted subcutaneous xenograft tumors.All ofthese xenografts were categorized as serous adenocareinomas, overexpression of CA_(125)and cytokeratin-7 which were original tumor phenotype of ovarian cancer. Conclusion Thesphere-forming cells isolated from primary ovarian cancer tissues have thecharacterization of cancer stem cell and may be a more reliable model system forunderstanding the biology of primary human tumors.