How do institutions respond to expert contests over epistemologies of sex and gender? In this article, I consider how epistemological ascendancy in debates over the regulation of women athletes with high testosterone is established within a legal setting. Approaching regulation as an institutional act that defines forms of embodied difference, the legitimacy of which may be called into question, I show how sexed bodies are enacted through and as part of determinations of expertise. I focus on proceedings from 2015 when (...) the Court of Arbitration for Sport was asked to decide whether an Indian sprinter, Dutee Chand, could compete as a female athlete. Despite acknowledging that sexed bodies are unruly, the court ultimately endorsed the use of testosterone as seemingly essential to women’s athletic performance, thereby reasserting a two-category model of biological difference. The legitimacy of these regulatory efforts was established through the concurrent narrowing of expertise and the body, a process that is also revealed to be gendered. (shrink)