Medicine, Founded by lhe Alpha Omega Fraternity, P. 0. Box 11 72, Jerusalem 91 01 0, Israel.We have recently shown that biniorph piezoelectric PVDF films induce formation of periosteal bone ~n uiuo and attributed this phenomenon to a piezoelectric effect. In the present study films were implanted in rabbits to encircle the femoral diaphysis. Specimens obtained after 6 and 12 days were subjected to routine processing for electron nlicroscopy as well as fixation using the Ka-pyroantimonate technique. The electron micrographs revealed that initial osteoblastic differentiation and formation of collagenous matrix were followed by Ca accumulation in mitochondria. Calcification of the matrix progressed with deposition of mineralizing nodules and their fusion to form larger calcified masses. Tlus was associated with disappearance of the pyroantimonate positive material from mtochondria. These ultrastructural observations confirm that bimorph films induce bone formation and disclose some features of the calcification process of the osseous callus.