Methods have been developed which permit electron optical examination of sections of undecalcified bone tissue which have been exposed only to organic solvents, thus minimizing artifacts induced in the mineral phase by aqueous solvents. Dense mineral granules are observed in the mitochondria of bone cells in unstained sections treated anhydrously, strongly suggesting that a solid phase of calcium phosphate exists in the mitochondria of these cells in vivo. Mineralization of the extracellular tissue spaces is first observed in the form of clusters or aggregates of small mineral particles. The very rapid increase in the size of the clusters and in the mass of mineral in the extracellular tissue space results principally from an increase in the number of additional particles formed rather than from the growth of the mineral particles first deposited. The physical chemical and biological significance of these findings is discussed.