Ambrose employs the so-called “formula of exchange” only once in his corpus, in his earliest work, De uirginibus. Arguing for virginity’s heavenly status, he writes: “A virgin conceived and the Word became flesh that flesh might become God.” In Ambrose’s subsequent writings the language of exchange and biblical passages related to deification (e.g. 2 Peter 1:4) are relatively rare; indeed, the terms deificatio and deificare are entirely absent from his corpus. This paper will discuss the “formula of exchange” in Ambrose’s De uirginibus and its place in the development of the bishop’s later thought on participation in the divine life. I argue that Ambrose’s anti-Homoian polemic alters his treatment of deification for two reasons. First, Ambrose aims especially to protect the Son’s unique equality (aequalitas) with the Father so that human deification remains derivative and respects the distinctive substantial union of the Trinity. Second, the bishop shifts the setting of deification from ascetical self-mastery to sacramental participation to emphasize the ecclesiological setting for union with Christ.