Summary. The purpose of the research is to analyze the formation and development of an international mechanism for the protection of national minorities under the auspices of the League of Nations in the context of its dissemination to the countries of Eastern Europe. The methodology of the research is based on the principles of historicism, science, author’s objectivity, consistency, social approach, and interdisciplinarity. General sciences (analysis, synthesis, deduction, induction, generalization) and special-historical methods (historiographical analysis, terminological analysis, retrospective, and comparative and historical) methods are used. The scientific novelty of the research is based on the analysis of a wide range of studies on the genesis of the protection of national minorities by the League of Nations, a critical assessment of the functioning of the League’s human rights mechanism in relation to minority ethnic groups in the countries of Eastern Europe. The research found that the system of guarantees of the League of Nations for the benefit of minorities was shaped under the influence and in the context of the development of international relations of the League with Eastern European states, at the stage of securing the respective obligations of the states, as well as during the supervision of their implementation. The article substantiates the existence of an alternative historiographical approach to the concept of «Eastern European states», which was characteristic of the interwar period, 20-s and 30-s of the 20th century, in the context of an urgent need for a systematic settlement of the national minorities’ status in Europe, which was of particular interest for the leading countries of the Entente.Conclusions. An international protection mechanism was primarily provided not to national minorities, but to social groups, as well as to their individual representatives. This significantly narrowed the rights and interests of «non-dominant» strata of the population of states. The system of minority rights guarantees of the League of Nations was also controversial: on the one hand, in many countries, there were significant violations of the rights of minorities, and, on the other hand, an established system often opened the way for possible abuses and artificial ethnic hostility agitation, such a way giving the possibility to a part of the country of origin of a particular minority to influence the policy of multinational states. The signatories to the treaty on the protection of minorities persistently resisted the desire of the League of Nations to provide effective control over the fulfillment of the treaty obligations imposed on them by the winner-countries of the First World War. They considered the created system inequitable and inappropriate since it disregarded the rest of the multinational states that faced similar problems. All this conditioned the League’s poor performance in protecting the rights and interests of minority ethnic groups living on the territory of Eastern European states.