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Objectives: Cell lines are valuable in vitro models for clinical and basic research. Most ovarian cancer cell lines described are serous cystadenocarcinomas or poorly differentiated adenocarcinomas. The establishment of ovarian cancer cell lines with rare histologic differentiation is especially of interest. We describe the establishment of a carcinosarcoma cell line of the ovary after in vivo selection., Methods: The cell line OV-MZ-22 was established from a solid tumor mass in the upper abdomen. At the time of establishment, the patient underwent secondary debulking and was pretreated with six cycles of cis-platinum/epirubicin/cyclophosphamide. Features of the cell line studied included morphology, ultrastructure, heterotransplantation, chromosome analysis, and analysis of intermediate filament proteins and actins by immunocytochemistry., Results: The first histologic report of the patient described a papillary cystadenocarcinoma, which changed to a carcinosarcoma with predominantly sarcomatous differentiation at secondary debulking. This cell line is aneuploid and shows no expression of the tumor-associated antigens CA-125 and CEA, but an overexpression of MDR-1, lung resistance protein, p53, and topoisomerase I and II, but not of multidrug-resistance-associated protein. The cell line did not give rise to transplant tumors in nude mice. The histologic and immunocytochemical comparison of the primary and the relapsed tumor proved evidence of an in vivo change of differentiation from predominantly papillary cystadenocarcinoma to carcinosarcoma. Morphological characteristics and intermediate filament pattern underlined the sarcomatous differentiation and origin of this cell line. The differentiation phenotype of OV-MZ-22 cells is that of smooth-muscle cells., Conclusion: The change of histologic differentiation was apparently due to a selection process caused by platinum-containing chemotherapy. The origin of the cell line and its rarity make this new line an appropriate tool for further investigation.