In the first months of the Spanish Civil War, the Spanish doctor Frederic Duran Jordà developed a new method of blood transfusion which overcame the era of direct arm-to-arm transfusions. While Duran was experimenting in Barcelona and the Aragon front, hundreds of foreign doctors came to Spain with the help of internationalist associations and offered their services to the Republican government. The Czechoslovak Dr Karel Holubec entered Spain in May 1937 and practiced in a mobile hospital funded by the Czechoslovak Committee to Aid Democratic Spain, receiving blood from Duran’s laboratory. This article aims to study how Duran and Holubec transferred the method of blood transfusion to Czechoslovakia through interpersonal contact, conferences, and performances. This paper argues that while individual actors played a crucial role in the diffusion of medical practices, this circulation was determined by a unique historical and socio-political framework. The Spanish Civil War, the International Brigades, and the invasion of Czechoslovakia by Nazi Germany were not only the historical context of medical innovation but an integral part of it.