Allogenic demineralized dentin implanted in the abdominal wall of quinea pigs induced heterotopic osteoid and bone formation. Samples of this tissue were frozen at–140°C, freeze-dried at -80°C, infiltrated with Spurr®epoxy resin and polymerized. The sections were then studied in the scanning transmission electron microscope, and analytical studies were performed by means of energy dispersive microanalysis.Osteoblasts and young osteoid osteocytes contained more calcium than undifferentiated cells and mature osteocytes. The Ca/P ratio in the cytoplasm of these young matrixproducing cells was high. Phosphorus was found in the nuclei of all cells and to a lesser extent also in the cytoplasm of cells rich in calcium. The initial extra-cellular mineral accumulation also showed a high Ca/P ratio. Small electrondense areas in these regions exhibited an amount of calcium and a Ca/P ratio resembling those of mature mineralized bone. The intial event of the mineralization process seemed to be an accumulation of calcium first in the young matrix-producing cells and then in the surrounding osteoid matrix.The results suggested that the matrix-producing cells are actively involved in the accumulation of calcium and phosphorus both intra- and extra-cellularly.