HEATING of bone to 325° C has been shown to induce the formation of pyrophosphate1-3. The quantity of pyrophosphate so formed is greater in the epiphysis than in the diaphysis and greater in the younger than in the older animal, and is therefore presumably greater in newly laid down than in older bone salt1. The question to be investigated is whether pyrophosphate-yielding mineral of new bone has specific chemical characteristics. There is much evidence which suggests that the tetracyclines are deposited in high concentrations in newly formed bone salt4. This communication demonstrates that the tetracycline labels the form of the bone salt that yields pyrophosphate in large amounts after heating.