The author reported in a previous paper that the method for enamel solubility (application of a cellulose acetate membrane disc (φ3mm) with the acid on the enamel surface and then measurement of calcium in the disc by atomic absorption spectrophotometry) had a great merit as an enamel biopsy.<BR>This article reports on the selection of the acid agent by studying both variation of the calcium dissolution and morphological changes of the enamel surface after acid agent application.<BR>1) Three acids (acetic acid, lactic acid and perchloric acid) and three buffers (sodium acetate-hydrochloric acid buffer, sodium lactate-lactic acid buffer and sodium acetate-acetic acid buffer) were studied about their enamel solubilities. Mean enamel solubility of a single tooth was determined from calcium amounts of the different locations on the buccal or labial tooth surface. Acid etched enamel surfaces were studied macroscopically and scanning electronmicroscopically. In addition, some of the surfaces were exposed to synthetic saliva for six days and then observed similarly.<BR>2) The mean enamel solubility showed a reverse relationship with the coefficient of variation, although the latter became plateau when the amount of dissolved calcium was more than 0.3-0.4 μg. The relationship may be expressed in the following fomula;<BR>y=ax^-bx: the calcium amount dissolved (μg)<BR>y: the coefficient of variation<BR>a; b: the constant of the each acid<BR>3) Coefficient of variation became plateau, when the dissolved calcium amounts of unerupted upper first premolar and erupted upper first premolar became 0.4-1.0 μg and when that of erupted lower central incisor became 0.3-0.4 μg.<BR>4) The macroscpic and scanning electronmicroscopic changes of the enamel surface appeared for the first time in the erupted upper first incisor at 1.5-2.0 μg of dissolved calcium amount, in the erupted upper first premolar at 1.5-2.0μg and in the unerupted upper first premolar at 0.5μg.<BR>5) When both erupted and unerupted teeth with etched enamel surfaces were exposed to synthetic saliva for six days, the etched enamel surfaces of erupted teeth did not show any changes scanning electronmicroscopically, but the etched surfaces of unerupted teeth revealed somewhate minute changes.<BR>6) Calcium amount dissolved by pH 2.3 sodium acetate-hydrochloric acid buffer was within 0.4-1.5 μg range in coefficient of variation plateau, and above this range, the changes of the enamel surface were recognized macroscopically and scanning electronmicroscopically.<BR>7) It was concluded from the present study that the method for enamel solubility, namely application of a cellulose acetate membrane disc (φ3mm) with pH 2.3 sodium acetate-hydrochloric acid buffer 2μl on the enamel surface and then measurement of calcium in the disc by atomic absorption spectrophotometry, had a significant merit as an enamel biopsy.