Human uterine cervical leiomyosarcoma cell line SKN has been proliferating stably for 11 years since establishment, while there has been no change in the cytobiological properties such as morphology (fibrous and long spindle), proliferating ability [doubling time (DT) = 30 hours, plating efficiency (PE) = 25%], or chromosome (mode: hyper tetraploid, large telocentric marker chromosome), etc. A part of the large telocentric marker chromosomes is found to be 2q as a result of G-band karyotyping, and the marker chromosome was recognized in all of the cells. Transplantation to nude mice was unsuccessful. 1 X 10(7) cells were planted on the chorioallantoic membrane (CAM) of chick embryo, but neovascularization was not evident. The conditioned medium of SKN cells slightly suppressed the proliferation of vascular endothelial cells. The reason for the unsuccessful heterotransplantation to nude mouse seems to be that neovascularization and proliferation of vascular endothelial cells do not take place.