Tracing the controversies surrounding commercial blood procurement in Japan, this article inquires into the reconfiguration of social boundaries, bodies, and substances through blood banks. In Japan, designated day laborers' enclaves, known as yoseba, became major pools of not only cheap labor force, but also of vital substances in the mid-twentieth century. Despite recurrent public health scandals, commercial blood banks continued to resurge in these districts, until they were completely replaced by the centralized donation-based Red Cross system in 1990. Analyzing media reports, published accounts, and policy papers, this article demonstrates how "sold blood" collected by commercial blood banks became the quintessence of "bad blood" in the process of this transition. Although blood donation is considered as an occasion to celebrate social solidarity today, this article shows that the specter of "bad blood" continues to haunt the body politic of Japan.