A urea-MFP mineralizing solution increased the average concentration of calcium in artificial plaque in 5 subjects from 7.6 to 219 μg/mg protein, but the concentration fell to 59.6 μg/mg protein during subsequent cariogenic glucose exposures. The concentration of phosphate rose from 5.4 to 107 μg/mg protein and then fell to 32.7 while fluoride rose from 0.008 to 7.62 and then fell to 4.27 μg mg protein. Following the cariogenic challenges, enamel beneath mineralized plaque showed an average increase in Knoop diamond indentation length of 4.3 μm (compared to 25.01 μm beneath unmineralized plaque) and no radiographic evidence of demineralization. Enamel softening was inversely related to the amount of mineral in the overlying plaque. When the maintenance was high, the development of experimental caries was virtually eliminated. Common ion suppression of enamel dissolution is the most likely explanation for the caries-protective effect.