The 5′-triphosphates of adenosine, guanosine, uridine, and cytidine inhibited the crystallization of calcium phosphate from stable supersaturated solutions that had been seeded with crystals of hydroxyapatite. Concentrations of the nucleoside triphosphates in the micromolar range were sufficient to have a significant effect on the crystal growth process. Diphosphate and monophosphate derivatives had less effect on the precipitation of calcium phosphate. It is possible that the nucleoside triphosphates may play a beneficial role in regulating intracellular calcification, particularly in disease states in which calcium entry into cells is pathologically high.