The immunolocalization of periostin, previously termed osteoblast-specific factor 2, was investigated in developing long bones of 17-day-old fetal mice and of 1-, 2-, 3- and 8-week-old mice at the light and electron microscopic levels. Fetal femurs showed immunoreactions for periostin in the periosteum, perichondrium, articular surface of the epiphyseal cartilage, joint ligaments, and fascias of surrounding muscles. In particular, intense immunoreactivity for periostin was found in the fibrous layer of the periosteum and perichondrium. At postnatal 1-and 2-weeks, in contrast, the immunoreactivity was restricted to the periosteum and thick fascias of surrounding muscles when compared with the fetal bones. Immunoelectron microscopic observation of the periosteum demonstrated immunoreaction products for periostin at the junction of periosteal fibroblasts and collagen bundles, suggesting its competence in the cell-to-matrix interaction. Mice at 3 and 8 weeks, unlike 2-week-old mice, showed a periostin-immunoreaction dominantly in the osteoblastic layer but not in the fibroblastic layer of the periosteum. Furthermore, the perichondrium and fascias of surrounding muscles were devoid of immunoreaction. Thus, periostin was confirmed to be widely distributed in bone and concomitant tissues at the fetal stage. As mice grew, however, its immunoreactivity gradually came to be restricted to the osteoblastic layer of the periosteum. Our findings suggest that periostin acts at the site of the cell-to-matrix interaction in periosteum, fascias, and joint ligament during morphogenesis of these tissues.