The component of Helicobacter pylori responsible for mouse death was identified and partially characterized. Mice that were injected with H. pylori cell lysate showed pathological changes such as decreased activity, diarrhea, mild convulsion, dramatic decline of body temperature, and even death. In order to identify the lethal factor, recently isolated H. pylori strain 335 and old culture (for 10 years) of H. pylori strain 51 were used. $LD_{50}$ of the cell lysate of H. pylori 335 and 51 were $338{mu}g;and ;985;{mu}g$, respectively, meaning the long passage of H. pylori in the laboratory might have decreased the lethal activity in the lysate. Mouse lethal activity disappeared by either treatment of cell lysate with proteinase K or heating cell lysate at $60^{circ}C$ for 30min. Mutation analysis and genetic complementation study revealed that active urease of H. pylori is the mouse lethal factor. The recombinant H. pylori urease expressed in Escherichia coli showed similar lethal activity.