Apparatus and techniques were evolved for a sensitive and reproducible test measuring the demineralization by acid of hydroxylapatite or powdered enamel as the stationary phase in high-performance liquid chromatography columns. Demineralization was quantified by monitoring the column eluate for UV absorbance and integration of the area of the UV peaks, with concomitant serial calcium analyses. Treating the hydroxylapatite with sodium fluoride conferred relatively long-acting protection against demineralization. The main effect of the fluoride appeared to be in reducing the acid solubility of the outer layers of the hydroxylapatite. Contrary to earlier theories that the protective action of calcium glycerophosphate was exerted only in the media surrounding the teeth, this agent was found to interact directly with hydroxylapatite, reducing the extent of demineralization by an average of 50%. The UV and the calcium analysis data showed how the mechanisms of protection by glycerophosphates and fluoride differed.