Transplantation of faecal microbiota (FMT) is generally considered a safe therapeutic procedure with few adverse effects. The main factors that limit the spread of the use of FMT therapy for idiopathic inflammatory bowel disease (IBD) are the necessity of minimising the risk of infection and transfer of another disease. Obtaining the animal model of UC (ulcerative colitis) by exposure to DSS (dextran sodium sulphate) depends on many factors that significantly affect the result. intake of DSS with water is individual for each animal and results in the development of a range of various forms of induced UC. For this reason, the aim of our study was to evaluate the modulation and regenerative effects of FMT on the clinical and histopathological responses and the changes in the bowel microenvironment in pseudo germ-free (PGF) mice of the BALB/c line subjected to chemical induction of mild, moderate and serious forms of UC. The goal was to obtain new data related to the safety and effectiveness of FMT that can contribute to its improved and optimised use. The animals with mild and moderate forms of UC subjected to FMT treatment exhibited lower severity of the disease and markedly lower damage to the colon, including reduced clinical and histological disease index and decreased inflammatory response of colon mucosa. However, FMT treatment failed to achieve the expected therapeutic effect in animals with the serious form of UC activity. The results of our study indicated a potential safety risk involving development of bacteraemia and also translocation of non-pathogenic representatives of bowel microbiota associated with FMT treatment of animals with a diagnosed serious form of UC.