We have established and morphologically characterized a new human clear-cell sarcoma cell-line, HS-MM, from the pleural effusion of a 39-year-old man with pulmonary metastasis derived from the primary popliteal tumour. The HS-MM cells were round or spindle-shaped, with round nuclei containing extremely prominent nucleoli. Light microscopically, the heterotransplanted nude mouse tumours showed essentially the same features as those of the original sarcoma, revealing an alveolar pattern of tumour cells with abundant clear cytoplasm. Both in vitro and in vivo, the cells reacted with anti-S-100 protein and melanoma-specific HMB 45 antibodies by immunohistochemistry. Ultrastructurally, the cells contained round euchromatin-rich nuclei with large nucleoli revealing conspicuous nucleolonema, and a large amount of glycogen and a few lysosomal dense bodies, but no premelanosomes in their cytoplasm. The HS-MM cell line was thus fully proven to exhibit the unique characteristics of a clear sarcoma both in vitro and in vivo, being also compatible with an amelanotic melanoma. This cell line will therefore be extremely useful for clinicopathological and histogenetic studies on clear cell sarcomas.