Abstract The struggle over credit for the discovery of insulin serves as one of the ugliest examples of the tensions and rivalries inherent in the growth of large-scale laboratory science. An understanding of this historic controversy also lends insight into the use of informal self-narratives directed to the scientifıc community in the negotiation of credit for discovery. Such informal accounts constitute a distinct form of argumentative address that is separate both from formal oral and written presentations of research, also aimed at a scientifıc audience, and popularized accounts aimed at a lay audience. In providing informal self-narratives to their scientifıc peers, the four insulin principals discussed in this article apparently shared a tacit understanding of the narrative grounds on which they could base their claims for discovery credit. In this study, we uncover widespread thematic similarities within these narratives, possibly indicating a common set of criteria within the scientifıc community for judging not only formal scientifıc proof but also the narrative proof found in informal discovery accounts.