Pathogens have evolved an array of strategies to establish a productive infection. The extracellular proteins secreted by pathogens are one of unique mechanisms to evade the host innate immune response. Many secretory proteins transported by the bacterial secretion systems have been widely investigated inSalmonella. Certain extracellular nucleases are essential for bacterial pathogenesis. However, there is no current data available for the enzymatic properties of the proteins secreted bySalmonella. Therefore, in the present study we have identified and characterized the nuclease activity of the extracellular proteins fromSalmonella entericaserovar Typhimurium. It was demonstrated that the extracellular proteins fromS.Typhimurium exhibited the deoxyribonucleases activity against lambda DNA by agarose gel electrophoresis and agar plate diffusion method. The activity was observed at 16 degrees C, 37 degrees C and 42 degrees C, and found to be highest at 42 degrees C and inhibited at temperatures over 60 degrees C. The nuclease activity was stable under alkaline conditions (pH 7-10) and the optimum pH was 9.0. The nuclease activity was promoted at high ionic strength of Ba2+, Ca2+, Mg2+, and Ni2+. Nuclease zymography analysis revealed that there were four activity bands in the extracellular proteins; followed by LC-ESI/MS/MS analysis seven proteins were identified. As demonstrated by nuclease zymography, the recombinant 5 '-nucleotidase protein expressed in the prokaryotic expression system displayed the DNase activity. To our knowledge, the present findings represent the first direct and unambiguous demonstration of the nuclease activity of the extracellular proteins fromS.Typhimurium, and it provides an important fundamental for further investigation of the role of the extracellular proteins in pathogenicity and immune evasion.