1. The process of diaphyseal bone formation can be investigated by studying the rate of incorporation of radioactive precursors, administered in vivo into bone fractions of increasing density. 2. In the 4-week-old vitamin D-treated chick most of the osteoid becomes calcified within 12h and almost all within 2 days. The low-density calcified phase that is formed is converted into a higher density form and within 7 days the greater proportion of the calcified tissue is in the higher density form. 3. In the vitamin D-deficient chick of similar age the rate of calcification of osteoid is decreased, as is the rate of conversion into the higher density phase with the resultant accumulation of the lower density calcified form. 4. The higher density phase probably corresponds to hydroxyapatite and the lower density one to the ACP-pase described by Termine & Posner [(1967) Calcif. Tissue Res. 1, 8--23]. 5. The disorder in the process of calcification seems to be unrelated to the alteration in blood Ca2+ and phosphate concentrations, but related to the presence or absence of cholecalciferol.