The correspondence of the last king belonging to the Jagiellon dynasty with the Radziwill family is at present quite well known, chiefly thanks to Irena Kaniewska who published the majority of king Sigismund Augustus’ letters to the Radziwill family (Listy króla Zygmunta Augusta do Radziwiłłów, edition, introduction and commentary by I. Kaniewska, Krakow 1998). However, there are still 30 letters which have not been published. The majority of the unpublished letters are to be found in the Russian National Library in Petersburg, in the collection of autographs of Piotr Dubrowski – F. 971, op. 2, avt. 62. The letters of king Sigismund Augustus to Mikolaj Radziwill, the “Red-Haired” which are to be found there, constitute the basis of the present publication. Yet one has to bear in mind that it comprises only the correspondence in the Polish language. The letters which were analyzed in the present publication have a different character. The first three, dating back to the year 1548, do not contain any information of particular importance. They are associated in this way or another with the issue of king Sigismund Augustus’ marriage to Barbara Radziwill. The letter of 1553 contains Sigismund Augustus’ request addressed to Mikolaj Radziwill the “Red-haired” for advice concerning the efforts of tsar Ivan IV the Terrible to obtain the royal crown; the latter efforts were undertaken by the tsar in Rome and had won the support of the emperor. Undoubtedly, the most interesting letters are those which were written in the year 1556. They concern the conflict over the Polish Inflanty (Livonia) and the military campaign against it which was being prepared at that time by the Polish king; it was Mikolaj “Rudy” (the Red-haired) who was to be the commanding officer in this campaign. The above letters furnish an answer to the question why no military conflict had taken place then.