At the beginning of the 20th century, research on material substances such as blood and metals was in high demand, as is apparent from the successful careers of the serologist Ludwik Hirszfeld (1884–1954) and the metallurgist Jan Czochralski (1885–1953). Both were leading experts of their time, their transnational biographies – spanning the German-speaking countries and Poland – were remarkably similar, and they both played important roles in the development of their respective disciplines. This paper explores how their contributions were closely (...) tied to the circulation of their respective research materials – blood and metals – and of knowledge about them throughout Europe and beyond. These examples forcefully demonstrate that, in order to fully understand how specific, widely accepted bodies of knowledge emerge, both local situations and the material conditions have to be considered. Drawing on Latour's actor-network theory, the transnational stories of serology and metallurgy demonstrate the range of human and non-human actors, practices, and techniques that contributed to the circulation of knowledge, to its successful application, and also to occasional failure in specific geographical and cultural contexts. (shrink)