In this article I attempt to analyze a Coptic manuscript sa 52 (according to the nomenclature of K. Schüssler), from the collection of Pierpont Morgan Library. In this parchment codex we have almost the entire text of the Book of Isaiah in the Sahidic dialect of the Coptic language. The first part of this book is particularly important: the so-called Proto–Isaiah. One third of the text of Proto–Isaiah in Sahidic can be found only in our manuscript sa 52! The presentation and analysis of the codex sa 52 is divided in several parts. The first part contains a short history of our manuscript against the background of the whole Pierpont Morgan collection. In the second part there are some technical informations about our codex (its form, measurements, shape of the letters, manner of writing, etc.). The third part is dedicated to the Coptic text itself and its comparison with the Greek text of the Septuagint (according to the edition of J. Ziegler). In the last part – based on several examples – we try to answer the following question: how the Coptic text of sa 52 helps us in our interpretation of the Greek text of Proto–Isaia, especially when the text of Septuagint is difficult and unclear.