The diagnosis and classification of soft tissue sarcomas can pose difficult problems for the histopathologist. Many sarcomas are too poorly differentiated to exhibit morphological features specific enough to define their histogenesis. Using the immunoperoxidase technique with commercially available antisera as a routine adjunct to other diagnostic aids, it is possible to arrive at more accurate diagnoses on which treatment protocols can be based. In addition a better understanding of mesenchymal neoplasms and their origins can be obtained by functional immunohistochemical studies.