ENThe first part of the article presents the instructions of the operational work of the Communist Security Service in the Polish Peoples’ Republic (PRL). The author introduces the organization and functioning of agencies: discusses the mechanisms for selecting candidates for the secret collaborators, principles of obtaining secret collaborators and rules of co-operation with them as well as the types of operational issues. The institution became an important mean of terrorizing the residents. It provided information (usually on the basis of denunciations), used to repress individuals and groups conducting “hostile activities” or suspected of such activities. There were no statements which specified the meaning of the term “hostile activities”; therefore the Security Service (SB) had the possibility of its unlimited interpretations. The Security Service (SB) was interested in the Lithuanians residing in Punskas and Seinai regions. Any of the activities were classified as “hostile activities”, therefore SB interfered in their work and private life, applied repression, accused the Lithuanians of nationalism. They tried to recruit secret collaborators among the Lithuanians. The most active of Lithuanians were recorded and had operational matters. The article provides the examples of the work of the Security Service in the territories where Lithuanians lived. The information is taken from the materials from the archives of the Institute of National Remembrance in Białystok.