By light microscopy the subdermal nodule of a patient with fibrodysplasia ossificans progressiva (FOP) had a fibromatoid histologic appearance. The cytoplasm of the cells stained strongly for mannose-rich glycoprotein with the concanavalin A-horseradish peroxidase (con A-HRP) method. The tumors also exhibited abundant hyaluronidase-digestible mucopolysaccharide in the interstitium with various basic staining reagents. This material appeared to consist principally of hyaluronic acid or chondroitin sulfate with few or mainly masked sulfate esters. At the ultrastructural level, cells interpreted as the tumor cells in the subdermal nodule from the patient displayed extremely hyperplastic granular reticulum and well-developed Golgi elements and appeared very active in synthesis and secretion of protein. The material in the dilated cisternae of the granular reticulum stained for glycoprotein with the con-A-HRP method. Macrophages which comprised the other main cell type in the nodules commonly contacted the tumor cells and occasionally evidenced engulfment of these cells. The intercellular matrix of the nonossified subdermal nodule exhibited greatly increased mucosubstance and, by electron microscopy, showed an unusual network of dialyzed iron-reactive acid muco-substance in the interstitium.