This chapter examines the reception of classical constructions of bodily fluids, especially blood, in Medieval and Early Modern martyrologies. It analyses how the martyrologists employed classical medical thought and early Church beliefs in response to their own concerns. New Testament depictions of blood and martyrdom and the discourses of martyrs’ blood found in early-Church writers, such as Origen and Tertullian, were the foundations upon which later martyrologists built. They employed and adapted this heritage to suit present concerns. Reformation martyrologists utilised martyrs’ blood as a polemic weapon, rival confessions seeking to align themselves with what they portrayed as early-Church precedent. At the root of these divergences lay fierce disagreement over the exact nature of early Church theology of the martyr’s body, a perspective this chapter will open up. In contrast to this overt engagement with early-Church sources, classical medicine is usually only implicitly present in Medieval and Early Modern martyrologies. Nonetheless, analysis of their use of classical medical thought offers valuable insights into changing theologies of the body. Medieval martyrologies drew upon classical medical thought in constructing the martyr’s body as spiritually fertile and as miraculous, for example depicting martyrs ‘bleeding’ milk. As the spectacularly miraculous disappeared from Early Modern martyrology, so too did many echoes of classical medicine. Some remained, however, both in metaphors emphasising the life-giving nature of martyrdom—such as imagery of martyrs’ blood as milk, sperm, or maternal blood—and in paralleling of blood and tears. The Medieval and Early Modern sources examined in this chapter include three popular Medieval hagiographical compilations (Jacobus de Voragine’s Golden Legend (c. 1260), John Mirk’s Festial (c. 1380s), the anonymous fifteenth-century Speculum Sacerdotale, works by the leading English Protestant martyrologists John Bale and John Foxe, and texts by key English Catholic apologists, such as William Allen and Robert Parsons.