Monophasic synovial sarcomas of spindle-cell type and fibrosarcomas were studied by electron and immunofluorescence microscopy for their intermediate filament expression and the binding of peanut agglutinin (PNA). In monophasic synovial sarcomas of spindle-cell type (two cases), frequent cell-to-cell junctions, irregular cytoplasmic processes, and occasional cytoplasmic, tonofilament-like bundles of intermediate filaments were seen by electron microscopy. These features were absent from fibrosarcomas. Immunohistologically, the monophasic synovial sarcomas showed arrays of prekeratin-positive cells in the midst of the vimentin-positive spindle cells. By double fluorescence microscopy, the prekeratin-positive cells also bound PNA, like the epithelial-like cells of the classical biphasic synovial sarcoma. In contrast to monophasic synovial sarcomas, prekeratin-positive cells and arrays of PNA-binding cells, were not seen by immunofluorescence microscopy in fibrosarcomas (seven cases). Thus the prekeratin-content, the binding of PNA lectin, and certain ultrastructural features suggesting early epithelial differentiation, help to distinguish monophasic synovial sarcomas of spindle-cell type from other spindle cell sarcomas.