Correlated light and electron microscopic study of four fibrous histiocytomas of proven malignancy has confirmed the presence of histiocyte-like and fibroblast-like cells in this tumor. In addition, an undifferentiated cell type, giant cells, xanthomatous cells, and rare cells with morphological characteristics intermediate between those of histiocytes and fibroblasts were seen. “Nuclear body” type inclusions were commonly present in both principal cell types in all four cases, as were the somewhat less common cytoplasmic inclusion bodies. Electron microscopic study of a tissue culture of one of these tumors demonstrated only fibroblast-like, histiocyte-like, and xanthomatous cells. The possibility is suggested that both principal cell types in this tumor may derive from the same undifferentiated stem cell.