Lactobacillus casei Zhang,isolated from traditional home-made koumiss in Inner Mongolia of China,has been considered as a new probiotic bacterium for its characteristics,especially acid tolerance.Proteomics researches were carried out to identify proteins expression of Lactobacillus casei Zhang grown at different pH values of pH 7.0 and 5.5.Cytosolic proteins of mid-exponential phase were resolved and further analyzed by two-dimensional gel electrophoresis.As results shown,eleven protein spots were found showing statistically significant differences.Three of total protein spots were identified as EF-Tu,NagA and sHsp using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-TOF/MS),indicating the complex response of lactic acid bacteria to acid stress.