Proteins of intermediate-sized filaments and desmosomal plaques (desmoplakins) of four human ovarian granulosa cell tumors were studied by immunofluorescence and immunoelectron microscopy and by two-dimensional gel electrophoresis of microdissected tissue samples. All tumor cells, irrespective of their specific histologic patterns, contained both vimentin and desmoplakins. Cytokeratin-positive structures were absent or very scant in most tumor regions, but more common in trabecular, insular, macro- and microfollicular structures. Biochemical analysis revealed the presence of Cytokeratin Polypeptides 8 and 18. Desmin filaments, neurofilaments, and glial filaments were not detected. Immunoelectron microscopy showed vimentin filaments attached to desmoplakin-positive plaques of desmosomes. These results indicate that granulosa cell tumors contain true desmosomes, which are associated primarily with vimentin filaments. This phenomenon has so far only been described in meningiomas and in blastema cells of nephroblastomas. Our observations suggest that in most neoplastic granulosa cells one epithelial feature, ie, cytokeratin expression, is greatly reduced, whereas desmosomes are still formed in appreciable frequencies. This unusual constellation of cytoskeletal elements in granulosa cell tumors may be useful in the differential diagnosis from other ovarian neoplasms, especially undifferentiated carcinomas. The importance of the use of antibodies specific for exclusively desmosomal proteins in classifying morphologically ill-defined junctional structures (eg, "rudimentary junctions," "primitive junctions," "desmosome-like junctions") is emphasized.