Two undifferentiated (embryonal) sarcomas of liver were studied ultrastructurally and immunohistochemically. Electron microscopic examination of the pleomorphic tumor cells revealed fibroblastic and histiocytic characteristics. There were no specific findings to support rhabdomyoblastic, leiomyoblastic, or epithelial differentiation. Cytoplasmic peroxidase-antiperoxidase (PAP) immunohistochemical staining for vimentin, alpha1-antitrypsin, and alpha1-antichymotrypsin was found. No staining for epidermal or internal organ cytokeratins, desmin, myoglobin, or alpha-fetoprotein was observed. The ultrastructural correlates of the cytoplasmic periodic acid-Schiff-positive, diastase-resistant hyaline globules were large, membrane-bound, heterogenous electron-dense inclusions, probably lysosomal in origin. These inclusions did not react on either alpha1-antitrypsin or alpha1-antichymotrypsin PAP staining. Tumor specimens from two metastatic sites were also examined. Neither contained the ducts or cysts that characterized the primary tumor. These studies confirm the mesenchymal nature of this uncommon childhood neoplasm and support the suggestion that the cytoplasmic hyaline globules represent a degenerative phenomenon. There are ultrastructural and immunohistochemical similarities with malignant fibrous histiocytoma.