Unusual intranuclear inclusions in malignant fibrous histiocytoma: presence in primary tumor, metastases, and xenografts.

We describe previously unreported intranuclear inclusions in 2 cases of malignant fibrous histiocytoma. The inclusions were found in 2-10% of the tumor cells removed from the patients and in 2-10% of the cells examined in tumor tissue xenotransplanted in nude mice. By stereo electron microscopy the inclusions are closely packed undulating fibrils 18-23 nm in diameter. They are sometimes associated with fibrillary bodies. They closely resemble the inclusions reported in some animals inoculated with serum from patients with non-A non-B hepatitis; however, their nature at present is unknown.