Keratin subsets in spindle cell sarcomas. Keratins are widespread but synovial sarcoma contains a distinctive keratin polypeptide pattern and desmoplakins. The presence of individual keratin polypeptides and desmoplakins was immunohistochemically studied in 25 spindle cell sarcomas of different types using acetone-fixed frozen sections. Results revealed that keratins 8 and 18 were present in a high number of tumors: 9 of 9 synovial sarcomas, 5 of 7 leiomyosarcomas, 5 of 5 malignant schwannomas, and 1 of 4 undifferentiated spindle cell sarcomas. In addition to keratins 8 and 18, the glandular component of synovial sarcoma showed prominent reactivity with antibodies to keratins 7 and 19. Also the glandular epithelial cells in synovial sarcoma showed desmoplakin immunoreactivity preferentially in a luminal distribution, but desmoplakin was absent in other spindle cell sarcomas. Furthermore keratin 13 was seen focally in 4 of 9 synovial sarcomas. In contrast, keratins 7, 13, and 19 were practically absent in leiomyosarcomas, malignant schwannomas, and undifferentiated spindle cell sarcomas. The widespread presence of keratins 8 and 18 in various spindle cell sarcomas may reflect aberrant keratin expression in mesenchymal cells, previously described in cultured transformed fibroblasts. The presence of keratins 7 and 19 and desmoplakin is highly associated with morphologically observable epithelial differentiation restricted to synovial sarcoma among spindle cell sarcomas. 