For an enamel solubility test as screening procedure of caries susceptibility (activity), the follwing five conditions are required : Ideally, the test should be, (1) applicable to all tooth enamel surfaces, (2) easy to perform, (3) simple in analysing its result (4) minimum in damaging the enamel, and (5) known about its accuracy.The present study was performed to find a method with above five conditions. The following results were obtained.1) Sensitivity of calcium by atomic absorption spectrophotometry (AAS) was 0.05 (1% absorbance value) μg/ml of 0.005 (2N=S value) μg/ml.2) The linear range of phosphorus standard curve by AAS of molybdenum was 0.3-0.5μg/ (n-butyl acetate) 5 ml (Air-Acet. flame) and 0.1-1.0 μg/ (n-butyl acetate) 5 ml (Nitrous-Acet. flame).3) The determination of calcium by AAS was interfered with the presence of sodium fluoride, EDTA, albumin and phosphate. Such interference was overcome when 3 mg/ml of lanthanum was added to the sample.4) The amounts of calcium and phosphorus obtained from the enamel surface by a filter paper disc containing perchloric acid changed with volume, concentration and application time of the perchloric acid, and pretreated condition of the enamel surface by various polishings. It was also found that calcium/phosphorus ratio changed similarly.5) Scanning electronmicroscopy showed that the amount of dissolved enamel correlated with the roughness of the etched enamel surface.6) When a filter paper disc and a cellulose acetate membrane disc were compared with each other, the cellulose acetate membrane disc was found to be more suitable as an applicating material than the filter paper disc, since the latter contained a small amount of calcium.7) The amounts of calcium, dissolved from enamel surfaces after application of the sixteen agents, were investigated. Acid buffers (pH 4.0 sodium acetate-acetic acid and others) showed a highest amount of calcium dissolution, organic acids (acetic acid, lactic acid, tartaric acid and others) an intermediate, and inorganic acids (hydrochlyric acid and perchloric acid) alowest.8) When the dissolution of calcium was less than 3-4 μg, the enamel surface was still intact macroscopically.9) It was concluded from the present study that the method for enamel solubility, that is, application of a cellulose acetate membrane disc (∅3 mm) with the acid on the enamel surface and then measurement of calcium in the disc by AAS, satisfied three (1, 2 and 3) of the five conditions mentioned above.