Iron uptake by osseous tissue was investigated by immersing frozen sections of fetal mouse bones in selected solutions at pH 7.4 and evaluating the changes microscopically. Iron uptake occurred when the phosphate concentration of the solution was normal and the calcium ion concentration was zero or very low compared with the concentration in interstitial fluid. The uptake was associated with decalcification of the sections. This low-calcium decalcification was not caused by a decrease in pH or by chelation of calcium ion, but seemed rather to be a simple dissolving of the bone salts. When the calcium ion concentration was normal or high iron uptake did not occur and the sections did not decalcify. When the calcium ion concentration was high and the phosphate concentration was normal or elevated supercalcification was observed wherein the trabeculae were thickened by a deposit of crystals resembling those in the normal bone. It appeared that when iron uptake occurred the iron was replacing calcium in the inorganic compartment of the bone, probably in the form of ferric phosphate.