R ESULTS are reported in this paper of a series of experiments related to an investigation of the underly ing factors in destructive changes in the solid (calcareus) tooth structures in the mouth. In an earlier report it has been pointed out that the dissolution of the constituents of the teeth may, in part, be accomplished thru alteration in the acidbase balance and this would be the case whether referring to the dentin, cementum or enamel. It is apparent that the alteration o f the oral aspect is not re lated to the activity of living cells, such as has in some cases been assumed, e.g. the absorption o f bone by osteoclasts, but it is recognized that bacterial metabolism in the mouth may affect the hydrogen ion concentration, an increase in which would tend to bring about solution of the enamel. The action of no specific acid is indicated but, if the dissolution of the enamel (as in caries) is typical of acid action, in which histopathologists appear to concur, the hydrogen ion concentra tion would bear some relation to the process. This question has been investi gated. It has not been practicable to ascertain the true acidity of the various types o f mouth fluids, altho this is a field which would result in considerable profit if investigated under the proper con ditions. The hydrogen ion concentration of the buccal fluid in health has been observed by Clark and Lubs, Mich'aelis and Pechstein, and probably others, but, without additional observations regard ing related factors which affect the fluid, this information is not satisfactory in the study of lesions o f the teeth. Our approach has been to observe the changes in dentin and enamel in known solutions of different hydrogen ion concentration and in artificial bacterial cultures. The experimental results indicate that the tooth material is affected by certain hydrogen ion concentrations but, it is im portant to note, not in a region where litmus or phenolphthalein would be of much value. Ground or broken teeth crowns appear to act as a buffer and the rise of hydrogen ion concentration in bacterial cultures is slightly altered by its presence. N o noticeable rise in amino acid nitrogen has been observed in bac terial cultures to which had been added ground tooth material, as compared with control cultures. This may not, however, indicate that the dentin matrix is not digested thru bacterial proteolysis, since the organisms were studied in pure cul tures, a relatively short period of time only was consumed during each experi ment— not over three weeks— and, since bacteria utilize amino acids, their rise might not be detected bv the methods applied. A series of cultures o f some micro-organisms found in the mouth