The response of the facultative intracellular bacterium Francisella tularensis LVS to stress was assayed by pulse-labeling with [35S]methionine followed by two-dimensional gel electrophoresis and autoradiography. A temperature increase from 37 to 42 degrees C or exposure to 5 mM hydrogen peroxide induced increased syntheses of at least 15 proteins. Among these proteins were a 75-, a 60-, and a 10-kDa protein. By N-terminal sequence analysis, these three proteins were found to be extensively homologous to the highly conserved chaperone proteins DnaK, GroEL, and GroES of Escherichia coli. Antibodies specific to the DnaK homolog of E. coli reacted with the 75-kDa protein, and antibodies to the GroEL homolog of Legionella micdadei reacted with the 60-kDa protein. A readiness to respond to hydrogen peroxide with synthesis of the chaperone components may be fundamental to the intracellular survival of pathogens such as F. tularensis, which are exposed to oxidative stress while invading the host macrophages.