The reactivity of lysosomal enzymes in 17 fibrohistiocytic tumors were examined histochemically to evaluate the histiocytic nature of the tumors. The lysosomal enzymes examined were acid phosphatase (Ac-P), non-specific esterase (NS-E), beta-glucuronidase (beta-GL), and N-acetyl-beta-glucosaminidase (N-GA). The tumors examined were eight malignant fibrous histiocytomas (MFH) (i.e., five common types, two myxoid types, and one inflammatory type), one atypical fibroxanthoma of the skin (AFX), one dermatofibrosarcoma protuberans (DFSP), two giant cell tumors of the tendon sheath (GCT), and five dermatofibromas (DF). Seven of 8 MFH showed strong reactivity for all enzymes examined. Positive reactions were seen as evenly distributed granules in the cytoplasm of fibroblastic, histiocytic and giant cells in all types of the tumors. Almost all AFX, GCT, and DF showed moderate to strong reactivity. These findings suggest that these tumors were composed of cells with the enzymatic character of histiocytes. However, DFSP contained no cells giving a positive reaction, and thus its histiocytic nature could not demonstrated.