The thesis is devoted to the extensive, but under-researched manuscript tradition of Ruthenian and Old Church Slavonic translations of the early printed Polish hagiographic book Żywoty świętych by the famous Jesuit, the first rector of Vilnius University Piotr Skarga (first edition 1579, Vilnius). The total volume of texts translated from Skarga’s book by the Ruthenians of the Lithuanian-Polish Commonwealth is comparable (in respect of its large scale), perhaps, only to biblical translations. A source study approach was used in this research. After a survey of general information on the total number of texts in this corpus, its language and volume, the textual history of the Lives of St. Procopius of Caesarea (18 copies), St. Athanasius the Great (16), and St. Pelagia of Tarsus (15), which all reflect the linguistic and cultural features of the local “Cyrillic� translations from Żywoty świętych, was researched in detail, using the methods of textual criticism. In order to establish the translational nature of numerous Ruthenian and Old Church Slavonic manuscript versions, they were compared with the Polish printed text: the condition for their identification as a part of the corpus was their simultaneous linguistic and textual correspondence to the Polish original. The genealogy of numerous copies of the three texts was identified by comparing their textual features with their dependency on a specific edition of Żywoty świętych, published during Skarga’s lifetime. This manuscript tradition developed by the Ruthenians of the Lithuanian-Polish Commonwealth during the 17th–18th centuries is an example of interconfessional transfer of texts – lives of saints and sermons, popular in the Catholic environment for a long time, were adopted by the local Orthodox and Uniate communities. This happened no later than the early 17th century (in Skarga’s lifetime) in the Ruthenian lands of the Grand Duchy of Lithuania (not those of the Polish or Hungarian kingdoms). The Ruthenians translated 232 texts out of 422 texts (55%) contained in Skarga’s corpus. Most translations are attested in manuscripts of the Menaion type or those made entirely of translations from Żywoty świętych. This indicates a special need for Cyrillic extensive reading texts in the region. Ruthenian was the main language for these translations, while Old Church Slavonic was used for merely 44 texts. The Lives of St. Procopius of Caesarea and St. Athanasius the Great were translated into Ruthenian twice, while the Life of St. Pelagia of Tarsus — only once. Old Church Slavonic translations were made from earlier Ruthenian versions (and not directly from Polish). Unlike the creation of the Great Menaion Reader by Metropolitan Macarius of Moscow, this manuscript tradition was decentralized. Translations from Polish were probably made and disseminated mostly by Ruthenian parish priests, who felt a need for Cyrillic edifying church texts.