The paper includes the edition of the recently discovered second witness of the Letter of Patriarch Callistus to the Bulgarian spiritual leader Theodosius of Tǎrnovo and his associate Romanus (RGB, f. 98, Egorov N 274). The edition is accompanied by variant readings taken from Syrku’s edition of the text found in RNB, F I 211, which has been the only known witness of the document for more than a century. Both witnesses have a western Russian provenance and date to the second half of the 15th c. (the new one has recently been dated to the last quarter of the 15th c.). The witnesses were compared in terms of orthography, grammar and lexis in order to establish their connection to a supposed Bulgarian original. The detailed philological analysis allows us to conclude that though both witnesses show significant traces of the Second Slavic influence on Russian literature, the two scribes introduced the innovations in different ways and to a different extent. Taking all this into account we dare to suppose that both witnesses descend rather from an earlier copy-mediator with Russian spelling features than directly from a Middle Bulgarian original.