We describe here the establishment of a new synovial sarcoma cell line, SYO-1, derived from a biphasic synovial sarcoma that developed in the groin of a 19-year-old female. The cell line was maintained for more than 70 passages (more than 24 months) in vitro. The SYO-1 cells in monolayer culture exhibited a spindle shape without conspicuous pleomorphism. Immunohistochemically, the cells were positive for vimentin, type IV collagen, S-100, mdm2, bcl-2, c-Met and c-Kit. Tumors developed by their implantation in nude mice histologically showed biphasic features that were composed of areas of fascicles of spindle cells and areas of compact proliferation of polygonal to ovoid cells, which occasionally formed epithelial plaque and expressed cytokeratin and EMA. SYO-1 cells harbored the characteristic t(X;18)(p11.2;q11.2) translocation by chromosome analysis and SYT-SSX2 chimeric transcript by RT-PCR. The SYO-1 cells, the first characterized cell line derived from biphasic synovial sarcoma retaining the characteristic genetic and phenotypic features of the tumor, will be useful for various investigations on synovial sarcoma, especially for its epithelial differentiation.