During the late 1940s and 1950s, radioisotopes became important resources for biological and medical research. This article explores the strategies used by French researchers to get access to this material, either from the local Atomic Energy Commission (CEA) or from suppliers in the United States or United Kingdom. It focuses on two aspects of this process: the transatlantic circulation of both isotopes and associated instrumentation; the regulation of use and access by the administrative bodies governing research in France. Analyzing the (...) investigations conducted within laboratories associated either with the atomic energy agency or with the local National Institute of Health (INH), the paper discusses the part played by the new tools in the postwar transformation of biomedical research. It contrasts the INH successful development of biological studies and metabolic tracing with the mixed results of CEA in advancing cancer radiotherapy, thus highlighting locally defined "normal paths" to radiobiology. (shrink)