An established fibrosarcoma cell line (RFS) derived from a cadmium induced fibrosarcoma of rat and RFS-cells in different subcultures produced tumors in nude mice and baby rats which showed characteristics of fibrosarcoma, malignant fibrous histiocytoma and completely undifferentiated sarcoma. After thorough examination of cells in culture and in experimental tumors by light microscopic and electron microscopic methods that conclusion has been drawn that malignant fibrous histiocytoma can develop from fibrosarcoma. By this the assumption that malignant fibrous histiocytomas derive from tissue histiocytes is questioned. After the discussion of the general unspecifity of the storiform growth pattern and the cytology of malignant fibrous histiocytomas the idea is presented that many defined malignant soft tissue tumors inclusively fibrosarcomas may pass a phase which should morphologically be diagnosed as malignant fibrous histiocytoma. Conceding the practical value of this diagnosis we emphasize that the concept of the entity "malignant fibrous histiocytoma" should be critically reevaluated because of the probable heterogeneity of this tumor group.