Characterization of a newly established malignant meningioma cell line of the human brain: IOMM-Lee. A permanent malignant meningioma (MM) cell line of the human brain designated "IOMM-Lee" is reported. This cell line was successfully established from the tumor of a 61-year-old Chinese man with repeated recurrent primary intraosseous malignant meningioma of the skull. It has been subcultured for more than 60 passages during the past 30 months. The doubling time of cultured cells is approximately 62 hours. Tumorigenicity in athymic nude mice (Balb/c-nu/nu) who develop multiple pulmonary metastases was observed; the doubling time of tumor volume in vivo is approximately 5 days. Karyotypic analysis revealed this cell line to be of human origin and near-diploid, with a modal chromosome number of 49. The mesenchymal tumor marker vimentin and intracytoplasmic microfilaments were identified in the cytoplasm of tumor cells by indirect immunohistochemical peroxidase-anti-peroxidase assays and immunogold ultrastructural localization by transmission electron microscopy, respectively. Scanning electron microscopy of cultured cells and xenografted tumors revealed ellipsoidal or carrot-shaped tumor cells presenting a wrinkled surface with short sparse microvilli. Potential proliferating activity was determined by Ki-67 monoclonal antibody; the Ki-67 labeling index of cultured cells and xenografted tumors was approximately 36% and 30%, respectively. This newly established malignant meningioma cell line of the human brain may prove useful as a research model. 