The purpose of the article is to scientifically reconstruct the epistolary dialogue of P. Kulish with the mother of his wife Oleksandra Bilozerska – mother-in-law – Motrona Bilozerska. The research methodology is based primarily on the methods of archival and source studies, as well as on biographical and historical analysis (for the reconstruction of P. Kulish’s life and creative path and reproduction of chronological boundaries of correspondence between addressees), on the prosopographical method aimed at expanding and deepening biographical material by taking into account the human and personal in history. The scientific novelty lies in the analysis of the private correspondence between P. Kulish and M. Bilozerska. The letters are based on autograph copies stored in the Central State Archive-Museum of Literature and Art of Ukraine (Kyiv). Some of them are published for the first time. Conclusions. As a result of the correspondence analysis, the author of the article concludes that the image of P. Kulish emerges from the pages of these letters as a caring and loving husband, a good manager, a respectful son-in-law. The addressee embodies “everything worldly”. The letters of P. Kulish to M. Bilozerska also recorded biographical information, which is a valuable addition to the already known facts about the life of the Kulish- Bilozerska family at that time and to his own prosopographical portrait. As a rule, the writer’s letters to his mother-in-law are characterised by informative content because they were addressed to the family member. Besides, at that time, it was one of the most important means of communication. In the analysed correspondence, from the point of view of genre typology, the article singles out letters-requests, letters-confessions, letters-descriptions. In letters to his mother-in-law, the writer discussed with her not only everyday things using lyrical digressions, landscape descriptions, but also philosophical issues. This indicates the spiritual connection and high cultural awareness of the writer and his mother-in-law, who carried on correspondence presented in the article. Letters are submitted in the original language, with only minor spelling corrections.