This article discusses xenotransplantation (XTP: the surgical role of nonhuman tissues, organs, and cells for human transplantation) and examines the way its scientific promoters have defended their technology against potentially damaging public representations. The authors explore the criteria used to legitimate the selection of the pig as the best species from which to "harvest" transplant tissues in the future. The authors' analysis shows that scientists and medical practitioners routinely switch between scientific and cultural repertoires. These repertoires enable such actors to exchange expert identities in scientific discourse for public identities in cultural discourse. These discourses map onto similarities and differences between animal donors and human hosts. Finally, the case is used to comment on a number of related approaches where the dynamics of medical and scientific authority are discussed.