Forty cases of malignant fibrous histiocytomas (MFHs) consisting of 33 cases of storiform-pleomorphic subtype MFH and 7 cases of myxoid subtype MFH, were studied immunohistochemically. Immunohistochemical stains were performed by the labelled streptavidin biotin method. The primary antibodies used in this study were classified into 4 groups according to their main specificities: 1) epithelial and non-epithelial (mesenchymal) markers, 2) histiocytic markers, 3) myogenic markers, 4) and that considered as possible lipogenic marker. In addition to these markers, alcian-blue stain, periodic acid-Schiff stain, Masson-trichrome stain, silver impregnation stain, oil red O stain, and Sudan III stain were performed in some cases at the same time. Over 85% of the cases were positive for mesenchymal and histiocytic markers, and no cases were positive for epithelial markers. About 40% of MFH were positive for some myogenic markers, but none of these cases were positive for any skeletal muscle markers. S-100 protein, considered as a possible lipogenic marker through the results of other special stains, was positive in about 10% of the MFHs. The immunohistochemical results of myogenic and lipogenic markers were parallel to the histological subtype; the positive percentages for myogenic markers were about 50% in the cases of storiform-pleomorphic subtype, but were 17% in the cases of myxoid subtype. On the other hand, the positive percentage for lipogenic marker was only 4% in the cases of storiform-pleomorphic subtype, but was 50% in the cases of myxoid subtype. These were supposed to be additional and occult characteristics of MFH. Therefore, the interrelation between the immunohistochemical results and histological subtype of MFH interestingly suggested a new entity of MFH concerning the histogenesis of MFH: MFH was suspected to contain some groups of very poorly differentiated and/or dedifferentiated sarcomas and to be the precondition of some sarcomas.