To clarify the characteristics of the extracellular matrix of chondroblastomas, six cases were studied under the electron microscope, with special reference to proteoglycans and calcium in the cellular areas. In ruthenium hexammine trichloride (RHT)-stained sections the matrix was observed to be composed of rounded or polygonal fine granules and unbanded thin filaments that appeared to link neighboring granules together. Treatment with potassium-pyroantimonate showed intracellular accumulation of precipitates, mainly localized within the cisternae of the rough endoplasmic reticulum as well as in the extracellular matrix. The presence of calcium in the precipitates was confirmed using x-ray energy dispersive analysis. These findings, similar to characteristic features observed in calcifying systems, support the theory that chondroblastomas are of chondrogenic origin.