This paper reconstructs the early history of isospin up to and including its employment in 1951. 52 to conceptualize high-energy pion-proton scattering. Studying the history of isospin serves as an entry point for investigating the interplay of theoretical and experimental practices in early nuclear and particle physics, showing the complexity of processes of knowledge construction which have often been presented as straightforward both in physicists' recollections and in the historiography of science. The story of isospin has often been told in terms of the discovery of the first "intrinsic property" of elementary particles, but I will argue that the isospin formalism emerged and was further developed because it proved to be a useful tool to match theory and experiment within the steadily broadening field of high-energy (nuclear) physics. Isospin was variously appropriated and adapted in the course of two decades, before eventually the physical-mathematical implications of its uses started being spelled out. The case study also highlights some interesting features of high-energy physics around 1950: the contribution to post-war research of theoretical methods developed before and during the war, the role of young theoretical post-docs in mediating between theorists and experimenters, and the importance of traditional formalisms such as those of spin and angular momentum as a template both for formalizing and conceptualizing experimental results.