The ultrastructure of endochondral bone was studied using an aqueous solution of chromic potassium sulphate as the decalcifying agent. 0.5 mm thick sections of rat tibiae were fixed in buffered glutaraldehyde, immersed in an aqueous solution of 1% chromic potassium sulphate pH 3.4, dehydrated and embedded in Poly Bed 812 without exposure to osmium tetroxide. In unstained sections we observed clusters of crystal like structures throughout the osteoid and calcifying cartilage matrix as well as solitary needle shaped structures in association with collagen fibrils. Stained sections revealed nuclei, endoplasmic reticulum, membrane limited dense granules, mitochondrial particles and other cell components typical of bone cells. It appeared that the chromic potassium sulphate method preserves the relationship between hard and soft tissues well, gives fine cytological detail and produces images of intracellular and extracellular deposits identical to untreated crystallites. It is concluded that the chromic potassium sulphate method is indicated for ultrastructural studies of bone.