1. 1. Although the greater uptake by bone and bone mineral of 65Zn 2+ than of 47Ca 2+ as previously reported 1 was confirmed, it was also found that the fraction of bone Ca 2+ exchanged, due to the differences between isoionic and heteroionic exchange, was in some cases the same for the two radioisotopes. 2. 2. 65Zn 2+ was taken up by bone and bone mineral much more rapidly than 47Ca 2+ in the initial stages of reaction. However, as uptake continued, diffusion through bone appeared to become the rate-limiting process. Apparently this differed little for the two radioisotopes. 3. 3. The amount of Ca 2+ that entered the solution in these exchange experiments was equal to the amount of Zn 2+ that entered bone powder at pH 7.3 and anorganic bone at pH 6.0. Ca 2+ in solution was greater than Zn 2+ entering bone powder at pH 6.0, and less than Zn 2+ entering anorganic bone at pH 7.3. 4. 4. H + was shown to be a powerful competitor of Ca 2+ in exchange reactions, with a superiority of about 1000: 1. A mechanism was proposed to explain its behavior. 5. 5. Zn 2+ replaced Ca 2+ on the surface of bone mineral so rapidly that it protected the surface from attack by H + at pH 6.0. It thus blocked the mechanism of action of H +, suppressed much of the rise in pH that was found in its absence, and reduced to a considerable extent the amount of phosphate that entered the solution. These effects were interesting from the theoretical point of view, and are of potential practice importance. 6. 6. After bone powder or anorganic bone was shaken with solutions containing Zn 2+, stable Mg 2+ was found in the solutions in amounts up to 0.23 mmole/l. In some cases it was as much as 20% of the Zn 2+ that entered bone by heteroionic exchange. 7. 7. Ca 2+/P ratios of solutions which had been shaken with bone and bone mineral were either significantly greater than or significantly less than the ratios in the original bone samples and in theoretical hydroxyapatite, depending on conditions. These, and other data, suggest that the increase in Ca 2+ and P concentration in solution had little to do with solubility proper but was the result of chemical reaction. 8. 8. Bone powder and anorganic bone differed in quantitative aspects of their relations with 47Ca 2+, 65Zn 2+, H +, and stable Ca 2+ and P. Differences with regard to pH and rate of dissolution of bone mineral in the two forms were especially striking, and suggested that anorganic bone, as usually prepared, differed from bone powder not only in its lack of organic matrix, but in relative content of amorphous phosphate and crystalline hydroxyapatite as well.