The Letter concerning Enmities between the Kirillov and Iosifov Monasteries has been used since the middle of the nineteenth century as a major source for monastic relations of the early sixteenth century. It is a mid-sixteenth-century composition extant in a single copy. Most likely composed in the Volokolamsk Monastery by Vassian Koshka (d. 1568), the former archimandrite of the Vozmitsk Monastery, the Letter describes disagreements between Nil Sorskii and his follower Vassian Patrikeev, on the one hand, and Iosif Volotskii and his followers Dionisii Zvenigorodskii and Nil Polev, on the other. It is a polemical source intended to convince the reader that Nil Sorskii and Vassian Patrikeev provoked these disagreements. Containing inaccuracies, inexactitudes, and unconfirmable assertions, the testimony of the Letter differs from early sixteenth-century sources. It is not a reliable source of evidence for the early sixteenth century, but the point of view of its author is better understood within the context of Church-State and internal monastic relations of the post-Stoglav middle of the sixteenth century.