Seven cases of synovial sarcoma (SS), two with biphasic and five with monophasic histology, were studied immunohistochemically using monoclonal antibodies to intermediate filaments, keratin, and vimentin. Slender spindle cells and plump cells were constant components of all the tumors. Epithelioid cells were present only in biphasic SS. Epithelioid cells and plump cells were positively stained by both keratin and vimentin. Slender spindle cells were stained positive for vimentin and negative for keratin. Staining manner of each cell type was similar irrespective of monophasic or biphasic pattern. Present immunohistochemical studies suggested that monophasic or biphasic patterns in SS should be regarded as a different expression of the same disease. In addition, immunohistochemistry proved to be a useful tool to detect plump cells which were difficult to find on routine staining.