EARLIER work" 2,3 in this laboratory with the artificial mouth has attempted a qualitative demonstration of the etiology of the various process. More recently4 a semiquantitative method was reported in which changes in microhardness were used for measuring the rate of in vitro various attack. However, because of the variability of the rate of surface softening, the results from many teeth were averaged in order to establish trends.The present study was undertaken to determine the rate of change of enamel hardness under simpler conditions, with the purpose of demonstrating particularly whether variation in the resistance of teeth was an important factor. The rate of softening of teeth produced by a lactate buffer at a constant temperature and constant pH provides a simpler system than the artificial mouth to test the possible variability of individual teeth. This paper presents data on the rate of softening of human teeth exposed to 0.001 M lactate buffer at pH 5.0, 350 C. for periods up to 10 hours. The effect of topical fluoride on the rate of softening is also described. EXPERIMENTALHuman anterior teeth for this study were collected from dentists in Birmingham, Alabama, where the fluoride content of the drinking water is 0.0 to 0.3 ppm. After extraction, the teeth were stored in distilled water and thymol to prevent drying and to inhibit bacterial growth. Prior to measuring the hardness, the enamel surface was cleaned with water and a rubber prophylaxis cup, using very light pressure.Hardness determinations were made as described in a previous paper5 using a Kentron microhardness tester and a 500 gram load applied for 10 sec. Ten impressions were placed on each enamel surface at the beginning of the experiment and after each treatment period.* The average for these ten

THE purpose of the present investigation was to develop a method for measuring the rate of in vitro tooth decay. Since surface softening has been one of the initial signs of in vitro tooth decay, changes in hardness of the enamel surface might be a useful index of the rate of the caries attack. The value of a rapid quantitative method for evaluating anticaries agents is readily apparent.The "artificial mouth"' has been used in this laboratory to produce in vitro tooth decay, the various lesions produced being indistinguishable from those occurring in vivo. This has been shown by the general similarities' and by histologic,2 ultraviolet and polarized light3 studies of the lesions produced.The microhardness of intact surface enamel has also been investigated in this laboratory.4 It was shown that intact surface enamel varies in hardness on a single tooth surface as well as from tooth to tooth. The variation in hardness is generally within the range of 250 to 450 Knoop numbers. The inherent variation in hardness of a single surface poses a real problem when only slight changes in hardness are being measured. However, if a tooth is softened so much that its average hardness no longer falls within the normal range, then a definite softening effect will be demonstrable. EXPERIMENTAL Fig. 1 is a diagram of the apparatus used in the semiquantitive study of in vitro caries. The crowns of caries free, human, anterior teeth were cut into mesial and distal halves. The two halves were mounted, labial surface up, in an acrylic resin box (A) and held securely in place with inlay wax. Ten microhardness measurements were made on each half and the average hardness calculated for each tooth half.The tooth halves were then subjected to a various attack in the "artificial mouth" for 6 to 14 hours and their hardness redetermined. Bacteriologic medium, the same as used in previous work,5 containing 0.25 per cent glucose was maintained at a constant drip rate of 60 drops per minute. The medium was dripped onto a cotton muslin cloth (B) through which it slowly drained.Since some bacterial plaques made in the laboratory tend to develop faster than others, all plaques were pregrown for 24 hours in the "artificial mouth"