What does research with radioisotopes teach us about the relationship between science and the cold war? Did state patronage corrupt the freedom of scientific inquiry? Did security interests subvert the circulation of knowledge? Did 'big science' dominate research practices? Did US-Soviet rivalry shape the context in which research was done and knowledge shared? Was the 'peaceful atom' nothing but a vehicle for American hegemony? This paper will chart the trajectory of the use of radioisotopes in medicine, agriculture and biology from the late 1940s to the Atoms for Peace conference in Geneva in August 1955. It will use radioisotopes to trace the practices of cold war science that were made possible by the production and circulation of a particular research tool in the first decade after WWII. Our longitudinal study of the broad distribution and disparate uses of radioisotopes complicates the usual picture that the cold war impacted science by simply militarizing it. We will use radioisotopes to argue for a more nuanced picture of the interplay of knowledge, technology, and international relations in the atomic era.