The kinetics of growth on to calcium oxalate monohydrate and tooth enamel seed material have been studied in supersaturated solutions of calcium oxalate and calcium phosphate respectively. The rate of crystallization of calcium oxalate is proportional to the square of the supersaturation; the reaction appears to be surface controlled. Pyrophosphate, a frequently proposed renal stone inhibitor, markedly slows the reaction without affecting the overall kinetics. The mineralization of tooth enamel appears to involve octacalcium phosphate which forms in the early stages of reaction. Added fluoride ion maintained at a fluoro-stat controlled concentration accelerates the rate of calcium phosphate crystallization on tooth enamel substrate even when present at low serum levels. In contrast, the diphosphonates are effective inhibitors of mineralization. The kinetic results for calcium oxalate and phosphates are discussed in terms of the important problems relating to tooth mineralization and the origin and growth of renal calculi.