183 cases of soft tissue tumors were studied utilizing the immunoperoxidase technique to demonstrate alpha-1-antitrypsin, ferritin and lysozyme. The series comprises 50 malignant lesions, 34 intermediate malignancy lesions, 99 benign lesions of fibrohistiocytic origin, and 23 malignant tumors of non fibrohistiocytic origin. The actual results of the study are as follows: alpha-1-antitrypsin, ferritin and lysozyme were always absent in 10 fibrosarcomas, 2 liposarcomas, 2 Ewing sarcomas, 3 synovial sarcomas, 4 neurofibrosarcomas, and 2 rhabdomyosarcomas, but in 24 malignant fibrous histiocytomas, 34 cases of dermatofibrosarcoma protuberans and 102 benign fibrohistiocytic lesions, these activities were present in a percentage that ranged between 12% and 38% (average 25%). Differences in the frequency of positive reactions did not occur between benign and malignant fibrohistiocytic lesions. The immunohistological examinations carried out have, therefore, only a very limited value for the practical diagnostic evaluation, but, when positive, are important to clarify the histogenesis of the tumor.