KESEL et al.1,2,3 have stated that saliva from caries-free individuals, when incubated for eight days in a beef broth medium, produces a higher concentration of ammonia (estimated by the ‘Permutit’ method) than does saliva similarly treated from caries-susceptible subjects. They also stated that high concentrations of alkaline ammonium salts or Seitz filtrates of incubated caries-free saliva are lethal to the Hadley strain of Lactobacillus acidophilus, and conclude that high ammonia production in the dental plaque may be responsible for the natural freedom from caries which a few individuals enjoy.