Acetone-fixed frozen sections of 15 malignant melanomas of the skin with metastases were studied immunohistochemically for the presence of different types of intermediate filament proteins, synaptophysin, muscle cell actins, and desmoplakins. One of the melanomas was a primary toe tumor, and the others mainly regional lymph node metastases. The original diagnosis of melanoma was reconfirmed in each case, and the melanoma diagnosis of the metastases was verified by S100 protein immunostaining in all cases and by a monoclonal antibody to melanoma cells (NK1C3) in 7 cases. All melanomas were prominently vimentin-positive. In 10 of 15 cases, immunoreactive keratin could be demonstrated with antibody CAM 5.2. The presence of keratins was confirmed in selected cases with three other monoclonal antibodies including AE1, PKK1, and a monoclonal antibody specific for keratin number 18. Desmoplakin, another marker of epithelial differentiation, was not found in melanoma cells. Two melanomas contained neurofilament-positive tumor cells, which were however negative for synaptophysin. Desmin, muscle actins, and glial fibrillary acidic protein were not found in the neoplastic cells. On the basis of the present results one could conclude that the protein composition of the cytoskeleton of melanomas is more complex than has been previously thought and most importantly that melanomas may contain keratins.