Hyaline globules (extracellular collections of amorphous material) are identified in 10 of 59 renal cell carcinomas (RCC) and in 2 of 9 oncocytomas. Immunohistochemical characterization of these PAS-positive structures revealed the presence of basement membrane material in most cases. Collagen type IV and laminin were the predominant constituents, whereas fibronectin was detected only occasionally. Electron microscopic examination of the globules showed concentric multilayered accumulations of basement membrane material. No such structures were recognized in 8 renal pelvic transitional cell carcinomas nor in 2 metanephric adenomas. RCC associated hyaline globules were always negative for alpha1-antitrypsin (AAT), alpha-fetoprotein (AFP), amyloid A, cytokeratin, vimentin, or lysozyme. These features differ from those of the hyaline globules previously described in other malignant neoplasms, notably AAT-positive hyaline globules in ovarian tumors, and AFP-positive globules in yolk sac tumors. Identification and immunohistochemical characterization of hyaline globules in metastases may be helpful in determining the origin of occult primary tumors.