Francisella tularensis is a facultative intracellular pathogen. Its capacity to induce disease depends on the ability to invade and multiply within a wide range of eukaryotic cells, such as professional phagocytes. The comparative disinterest in tularemia in the past relative to other human bacterial pathogens is reflected in the paucity of information concerning the mechanisms of pathogenesis. Only a few genes and gene products associated with Francisella virulence are known to date. The aim of this study was to find and identify proteins of F. tularensis live vaccine strain induced in the presence of hydrogen peroxide, and to investigate the role of the IglC protein in the regulation of genes expressed upon peroxide stress. The [(35)S]-radiolabelled protein patterns were examined for both the wild live vaccine strain and its DeltaiglC1+2 mutant defective in synthesis of the IglC protein that was found to be strongly up-regulated during intracellular growth in murine macrophages in vitro and upon exposure to hydrogen peroxide. Globally, we found 21 protein spots whose levels were significantly altered in the presence of hydrogen peroxide in both the wild-type and mutant strains.