Embryonic Meckel's cartilage cells, following prolonged osmication, exhibited complete impregnation of the Golgi apparatus, endoplasmic reticulum (ER), nuclear envelopes, mitochondria, multivesicular bodies and some matrix vesicles. Most of the Golgi apparatus showed a specific localization of the reaction product. However, no deposition occured on the plasma membrane, in the cellular matrix and cartilaginous matrix. Osmium deposits were restricted within some type of matrix vesicles at the extracellular space. As to the stainability for the matrix vesicles, two different types were identified: one contained the reaction product, and the other was free of it. These results suggested that the intracytoplasmic organelles showing osmium-positive reaction might be involved in some way in the formation of matrix vesicles.