During the Second Sino-Japanese War, the technological project of mass immunization united state health administrations and international aid organizations seeking to prevent epidemics in unoccupied China's wartime hinterland. This article examines a joint wartime effort between the Chinese government's National Epidemic Prevention Bureau and the League of Nations Health Organization to manufacture and distribute vaccines against smallpox, cholera, and other diseases in northwest China. The hardships of war presented challenges to the development of large-scale immunization, but also led to the establishment of international aid programs that helped Chinese microbiologists acquire standard cultures, animals, and equipment. Vaccination provided a means for the beleaguered Nationalist government to quell epidemics and resist the Japanese; subsequent state involvement in the process of managing transport of vaccines, organizing and training vaccinators, and mandating the shots suggests the significance of mass immunization, as well as its reliance on technological systems in which vaccines embodied emerging biomedical standards that the state sought to institutionalize.