From the foregoing experiments, it is concluded that viability of the bone cells is not essential in the process of inorganic salt uptake by bone, provided the organic matrix is present and normal. The term "uptake", as used in this report, includes several possible processes: ionic exchange, recrystallization, and the formation of new crystals either in the old matrix or in conjunction with the formation of a new matrix. The latter can be demonstrated histologically in living tissue cultures of bone (Fig. 3). However, the amount of matrix formed over a ten-day period is probably insignificant in the over-all process of salt metabolism in tissue cultures, and, of course, need not be considered in the case of the dead bone (Fig. 4). Mobilization of the inorganic salts from bone, on the other hand, is facilitated by the presence of living cells. A corollary of these conclusions is that uptake of radio-active isotopes is probably not an accurate index of bone-cell "viability". This confirms the conclusion previously suggested by the work of Odell and his associates. A second conclusion reached from the present experiment is that the change in radiographic density in aseptic necrosis of bone is probably the result of combined failure of mobilization of inorganic salts and continued uptake by the necrotic bone from the tissue fluids, in addition to the decrease in density of the surrounding viable bone,—that is, the increase in density of the dead bone is actual as well as relative. An interesting sidelight on these observations is that bone-seeking radio-active emitters, such as radium, probably become relatiely fixed in the skeleton when radiation damage is sufficient to cause necrosis of the cells.