The article deals with less known aspects of the ecclesiastical and cultural activity of Metropolitan Cyprian, a Bulgarian by birth who stood at the head of the Russian Church in 1376–1406. On the basis of new sources and a more accurate dating of the handwritten tradition of the memorials, the author suggests that Cyprian took part in the translation of the life and service of the Lithuanian new martyrs in 1375 and in dissemination of the former monument in the Serbian literature. It is also suggested that Metropolitan and his chancellery bore on the dissemination of the most ancient epistolarium in Russia, translation of the famous medieval falsification which was justifying the Church’ claims on secular authority – the «Donation of Constantine», and spreading in the Metropolis of Kiev of a special early edition of the Serbian Nomocanon (Kormchaia Book), with a systematic composition of the material.