Purpose: This chapter provides a contextualized understanding of the gendered anxieties expressed by elite sport regulators that motivated the formulation of sex testing policies in sport between 1937 and 1968. The focus is on complicating the claim that sex testing was first instituted to prevent explicit male bodies from fraudulently masquerading as women in sport. Rather, the chapter argues that sex testing policies were formulated in response to anxieties over sex binary pollution. Methodology: The chapter is based on a genealogical study of the female category in elite sport, built on archival research conducted at the International Olympic Committee (IOC) historical archives and online newspaper archive collections. Findings: Boundaries around female embodiment were navigated and written into sex testing policy in response to threats to presumed ideas around gendered and sexed normality in sport. These threats were embodied by athletes who polluted or crossed the border between female and male, to the extent that their bodies were rendered hermaphroditic, excessively masculinized, or hybrid. These bodies caused gendered anxieties for sport regulators, who reacted with policy responses that aimed to purify the sex binary from category pollution or sex abnormality. Implications: As long as sex binary policing in elite sport continues, awareness of the contextual history of sex testing is essential for understanding the underlying ideas upon which sex binary policing in sport has been built.