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The aim of this work was to develop a standard quantitative method to measure the acid tolerance of probiotic cells when exposed to a simulated gastric fluid. Three model strains of different cell concentrations were exposed to a standard simulated gastric fluid of fixed volume. The fluid pH ranged from pH1.5 to 2.5. In general, the death kinetics followed an exponential trend. The overall death constant, k d, for all strains was found to be in a power relationship with the pH value and the initial cell concentration, and it can be expressed as kd= kAII( pH -9.0N 0-0.19 where k AII is defined as the acid intolerance indicator and N 0 is the initial cell concentration (CFU/ml). This equation was validated with the experimental data with an average R 2 of 0.98. The acid intolerance of cells can be quantitatively expressed by the k AII values, where higher value indicates higher intolerance. In conclusion, a standard quantitative method has been developed to measure the acid tolerance of probiotic cells. This could facilitate the selection of probiotic strains and processing technologies. © 2009 Springer-Verlag.