Antiserum raised against whole Helicobacter pylori cells identified a novel 94-kDa antigen. The nucleotide sequence of the gene encoding the 94-kDa antigen was determined, and analysis of the deduced amino acid sequence revealed structural features typical of the ClpB ATPase family of stress response proteins. An isogenic H. pylori clpB mutant showed increased sensitivity to high-temperature stress, indicating that the clpB gene product functions as a stress response protein in H. pylori.