^<47>Ca-Releasing activities of some vitamin D derivatives from prelabelled mouse calvaria were comparatively determined in a system of organ culture established by Raisz and Reynold. A rather high concentrations of ergocalciferol and cholecalciferol (1μg/ml) caused a significant release of ^<47>Ca from the bone, while dihydrotachysterol_2 and dihydrotachysterol_3 had no effect at the same concentration. Each of 25-OH-D_3,24R-OH-D_3 and 24,25-(OH)_2-D_3 at 100 ng/ml showed a significant ^<47>Ca-release. 1α-OH-D_3,one of synthetic analogues of active vitamin D, had more effective ^<47>Ca-releasing activities than these three natural derivatives. A concentration of 10ng/ml was enough to release ^<47>Ca from the bone. 1α, 24-(OH)_2-D_3 had a little less activity than 1α-OH-D_3,but more potent than 24-OH-D_3. These results suggest that the presence of the hydroxyl group at 1α-position causes a more effective release of ^<47>Ca than that of the 25- or 24R-hydroxyl group. Biological activities of vitamin derivatives were compared between R-and S-form in the same experimental system. Both 24R-OH-D_3 and 1α, 24R-(OH)_2-D_3 were about ten times as active as 24S-OH-D_3 and 1α, 24S-(OH)_2-D_3 in ^<47>Ca-release, respectively. 1α, 25-(OH)_2-D_3,natural hormonal vitamin D, showed the highest activity in ^<47>Ca-release in this organ culture system showing a significant activity even at a concentration of 50 pg/ml. The effect of 1α, 25-(OH)_2-D_3 on the release of ^<47>Ca from the bone was inhibited by eel calcitonin, but not by hydrocortisone, 25-OH-D_3 or 24,25-(OH)_2-D_3. This system may be used for assaying 1α, 25-(OH)_2-D_3 in human serum, and by changing the medium more frequently, the sensitivity of the assay will be increased to 5 pg/ml.
