THE tooth crown, once formed, is a stable calcified structure removed from direct equilibrium with body fluids and therefore undergoes minimal remodelling, exchange or turnover of mineral elements. During development and calcification, the mineral elements of the crown are in equilibrium with body fluids and, indirectly, with the diet. The concentration of strontium-90 in the tooth crown is, therefore, a measure of the equilibrium between the tooth and dietary strontium-90, and the tooth crown represents a permanent record of the equilibrium existing at the time of tooth formation.