Structural components of the organic bone matrix were located by immunohistochemical techniques in fresh-frozen sections of normal and dysplastic bone. Fine and coarse birefringent fibers were identified as separate and distinctive features in the extracellular matrix by antibodies raised against human collagen Type III. The glycoprotein tenascin was located on a proportion of the fibers in a characteristic beaded pattern, which was absent in dysplastic bone. The fibers originated in the periosteum or in the fibrous stroma of the marrow cavity and were oriented with regard to both the spatial and the lamellar organization of the bone. The disposition and composition of the fibers suggests that they form a preliminary framework on which intramembranous bone modeling proceeds, and that the specific location of tenascin on the fibers in normal developing membrane bone may be important in determining the alignment of the bone tissue. Epitopes recognized by the collagen Type I and fibronectin antibodies were demonstrated throughout the mineralized matrix, but their incorporation into the collagen "Type III" fibers was evident only outside the mineralized matrix.