Immunohistochemistry was used to examine 10 cases of malignant fibrous histiocytoma. Malignant cells in all cases expressed vimentin and in eight there was co-expression of either desmin or neurofilament, both of these being present in four cases. In addition, cytokeratin was found in one case. In each tumour, a population of small cells was identified which had the staining characteristics of benign macrophages, and this was distinct from the tumour cells. This study supports the concept that malignant fibrous histiocytoma is a tumour of mesenchymal cells rather than of histiocytes and emphasizes the diversity of its cytostructure.