An ultrastructural-morphometric study was carried out on the process of osteoid maturation in growing surfaces of parallel-fibered chick bone. The aim was to investigate the distribution, size and amount of collagen fibrils (CFs), as well as the proteoglycan (PG) content, throughout the osteoid seam and in the adjacent bone. The results show that the organic components secreted by osteoblasts undergo complete maturation inside the osteoid seam only. Proceeding from the secreting plasma membrane of osteoblasts (osteoidogenic surface) towards the mineralizing surface, we found that CFs gradually increase in diameter but not in number per surface unit. As a consequence, the proportion of osteoid seam occupied by CF increases too, at the expense of the interfibrillar substance. PG content also decreases inversely in this direction. In the adjacent bone, CF size and density do not change significantly with respect to the mature osteoid close to the mineralizing surface.