This paper investigates transnational circulations of scientific knowledge in the 1940s using the production of antibiotics in China as a case study. News of the therapeutic development of penicillin during the Second World War traveled quickly. In China, researchers at the National Epidemic Prevention Bureau (NEPB), led by the bacteriologist Tang Feifan and based in remote wartime headquarters in Yunnan, read about the drug in scientific publications and sought the means to produce penicillin domestically. Concurrently, in New York, the American Bureau for Medical Aid to China planned to directly transfer key technologies and personnel for penicillin production from the United States to China. While American advisers expressed qualms, seeing the poverty and destruction of wartime China as obstructing such a highly technical project, Tang and other NEPB researchers relied on these wartime conditions to identify relevant molds and other resources in the southwestern environment that enabled their success in penicillin production. They drew on multiple scientific networks, including not just American correspondents but individuals and institutions in Britain and especially British India, to obtain key information and materials. The paper argues that the Chinese effort to produce penicillin suggests the ways in which successful replications of experimental procedures across great distances depended on both the selective adaptation of materials, texts, and resources from elsewhere and local innovations—in surprising contrast to the failure of explicit American attempts to dictate the circulation of scientific knowledge as a physical transfer of texts, objects, and people.