Four cloned cell lines, MT-7, MT-8, MT-9 and MT-10, were established from a transplantable malignant fibrous histiocytoma (MFH) of F344 rats to investigate the histogenesis of the tumor. Cells of MT-7, MT-9 and MT-10 had fine structures characteristic of histiocytes, such as numerous cell processes, many lysosomes and well-developed cytoplasmic organelles. They stained positively for histiocytic lysosomal and antigenic markers. In addition, MT-9 cells contained microfilaments and well-developed RER in their cytoplasm, suggesting that they may be facultative fibroblasts. MT-8 cells stained weakly for histiocytic markers and had scant cytoplasmic organelles. They were identified as undifferentiated mesenchymal cells. The tumors induced in syngeneic rats by inoculating MT-7 or MT-10 consisted of a mixture of the pleomorphic, myxoid and storiform types of MFH, and those by MT-9 were of the storiform type. Cells forming these tumors stained positively for histiocytic markers. Tumors induced by MT-8 consisted of undifferentiated cells negative to these stainings. The histogenesis of MFH is surmised to be related to various differentiation stages shifting from undifferentiated cells to histiocytic cells capable of acting as facultative fibroblasts.