A cell line designated HTMMT was established from the human uterine carcinosarcoma (composed of leiomyosarcoma and adenocarcinoma) of a 66-year-old Japanese woman. The cell line grew well and 83 serial passages were successively done within 24 months. The cell line contained spindle- or fibrous-shaped cells that revealed neoplastic and pleomorphic features, and multipled rapidly without contact inhibition. These cells were characterized as possessing myofibrils. The karyotype exhibited hyperploidy and the chromosome number was ranged from 87 to 100. The cells were transplanted into an immune-depressed hamster cheek pouch or into nude mouse skin, but produced no tumors. We supported the combination theory for the histogenesis of the carcinosarcoma.