The effect of saliva on plaque acidogenesis was studied in ten caries-resistant and ten caries-susceptible subjects. Plaque pH was measured in vivo following exposure to a sucrose substrate under varying conditions of salivary access. Our findings demonstrate that when there is no salivary access, plaque pH levels are similar in the CR and CS groups. As the access to saliva is increased, the observed pH minima increased to a greater degree in the CR subjects than was noted in the CS subjects. This indicates that saliva (notably stimulated saliva) plays a major role in modifying plaque pH and quantitatively reflects caries status.