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 k(d)=k(AII)(pH(-9.0)N(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 kAII 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.