Histologic distinction between various fibrohistiocytic tumors of the skin may sometimes be difficult. Recently, several immunohistochemical markers of "histiocytes and "facultative fibroblasts" have been introduced and used for the study of some fibrohistiocytic tumors of the skin.

The purpose of this study is to determine whether immunostatining with MAC 387, antibodies to S-100 protein, factor Xllla(FXllla) and CD 34 allows distinction between various fibrohistiocytic tumors of the skin in formalin-fixed, paraffin-embedded specimens.

Paraffin-embedded specimens of dermatofibroma, keloid, hypertrophic scar, dermatofibrosarcoma protuberans(DFSP), neurofibroma, and juvenlie xanthogranuloma were investigated with MAC 387, antibodies to S-100 protein, CD 34 and FXllla using avidin-biotin -peroxidase complex method.

In all fibrous dermatofibromas (n=26), 20-90% of constituent cells were positive for FXllla. Focal or diffuse CD 34 reactivity was present in DFSP (n=2). Weak reactivity for CD 34 and consistent labeling for S-100 protein were found in neurofibromas (n=5). Tumor cells showed negative for FXllla, CD 34 and S-100 protein in keloids (n=2), hypertrophic scars (n=6), and juvenile xanthogranulomas (n=5). MAC 387 did not label tumor cells of the fibrobistiocytic tumors we have studied.

Immunostaining of paraffin-embedded specimens with antibodies to S-100 protein, FXllla and CD 34 may be useful in the differential diagnoses of fibrohistiocytic tumors of the skin.