The article refers to source materials in respect of the Polish bishops’ letter of 18th November 1965 to the German bishops, held in the archives of the National Remembrance Institute. Some of these materials are unknown to researchers. They are not significant enough to alter either our vision of the events related to the letter, or of its impact on the Polish-German relations; they may well, however, extend this knowledge to a substantial degree and complement it. These materials originate not only from the Polish special services, but also from the East German STASI.