The synovium is lined by a layer of intimal cells which have been classified on ultrastructural criteria into type A and B synoviocytes. The functionally important lining cells of the synovium(type A and B synoviocytes) are the subjects of many study but have presented problems with their characterization and microscopical identification. Synovial sarcoma is a distinct and generally recognized soft tissue tumor that its origin still raises controversy. In this study, 12 cases of nonspecifically proliferative and resting human synoviocytes have been obtained from the synovium of knee and hip joints, and 3 cases of synovial sarcoma which have arisen in the left thigh, left buttock and right inguinal region were examined with light microscopy, immunohistochemical observation, and electron microscopy. In light microscopic level, it was difficult to differentiate the type A synoviocytes from type B synoviocytes morphologically. The reactive type B synoviocytes were positive for the protein of cytoskeleton such as pancytokeratin, CK1, CK8, CEA, and vimentin. The resting type B synoviocytes showed positive reactions for pancytokeratin, CK1, and CK8. The markers for the monocytes/histiocytes(CD15, CD68, lysozyme, Al-AT, Al-ACT) were reactive in resting and reactive type A synoviocytes. Also, MHC class II antigen was reactive in type A synoviocytes. Three cases of primary synovial sarcoma were 2 fibrous monophasic and 1 biphasic. Spindle-shaped cell in fibrous monophasic synovial sarcoma showed reactivity for CK7 and pancytokeratin, and epithelial cells (lining the glands) in biphasic synovial sarcoma were reactive for CK 7, pancytokeratin, EMA, and focally CEA, but only spindle cells reactive for vimentin, By electron microscopy, fibrous monophasic synovial sarcoma showed pseudogland formation with intercellular junctions of paired subplasmalemmal density and discontinuous basal lamina. These results suggest that the reactive type B synoviocytes and synovial sarcoma show an aberrant expression of the vimentin and CEA. The expression of CK on the resting and reactive type B synoviocytes and fibrous monophasic and biphasic synovial sarcomas are different. Type A synoviocytes expressing the MHC class II molecule and monocyte/histiocyte markers suggest a member of the mononuclear phagocytic system. The reasons of the aberrant expression of the intermediate filament, vimentin and oncofetal antigen, and CEA in reactively proliferative type B synoviocytes and synovial sarcoma and the different expression of cytokeratin on the resting, reactive type B synoviocyte and synovial sarcoma should be further evaluated.