Mineralization of heterotopic bone was studied in a bone induction model using allogenic demineralized dentin implanted in the abdominal wall of guinea pigs. There was a high yield of newly formed osteoid and bone as well as some cartilage together with areas of resorption of the dentin, and fibroblast proliferation. The osteoid contained many matrix vesicles II and less of lysosome-like type I vesicles. Early cartilage formation had more type I vesicles. The implanted dentin contained no matrix vesicles. The first signs of mineralization occurred mainly as irregular clusters of mineral crystals in the matrix close to the surface of collagen fibrils. Crystal-like figures were also found inside some type II matrix vesicles, although most of these vesicles in the mineralization zone had no crystals. The type I vesicles of both bone and cartilage exhibited often crystals near the outer membrane. The mineralizing bone showed a reduction in the size and number of proteoglycan particles. Remineralization of the implanted dentin was also often found and the mineralization pattern resembled the mineralization of bone except for the absence of matrix vesicles. Electron diffraction of selected areas showed that the crystals in the new bone and the mineralized dentin were hydroxyapatite.