This article reinterprets an incident that Livy (8.18.4–11) and derivative later sources place in the year 331 BCE: a wave of poisonings whose perpetrators are brought to light after an enslaved woman contacts a Roman magistrate. Its main objectives are to show that the incident is best understood in connection with the transmission of novel—or perceived as novel—pharmacological knowledge, and in conjunction with shifts in the institution of slavery at Rome that were set in motion by the Republic’s expansion; that (...) a key figure in the mythological encoding of this transmission was the legendary Circe; and that moving away from previous scholarship’s concern with the matronae alleged to have carried out the poisonings and focusing instead on “la servant délatrice” (Jean-Marie Pailler) opens up new corridors into the cultural history of this period. (shrink)