The relevance of this study is conditioned by the widespread use of stem cells in veterinary medicine, a wide range of studies and ambiguous data on the oncoprotective properties of stem cells of different origins. In this regard, the purpose of this study was to investigate the course of the tumour process in Lewis lung carcinoma and the specific features of the effect of allogeneic mesenchymal stem cells of red bone marrow culture on it. The leading approach to investigating this problem was the method of modelling Lewis lung carcinoma in C57BL6 mice and the use of stem cells. The use of allogeneic mesenchymal stem cells from the bone marrow culture of C57BL6 mice with transplanted epidermoid metastatic carcinoma of the Lewis lung contributed to the activation of the tumour process. Under the influence of allogeneic mesenchymal stem cells of red bone marrow culture from Day 14 to Day 24 of the study, the body weight of mice decreased by 7.0-12.1% (P < 0.05) compared to the control, the diameter of the primary tumour increased by 1.43-1.51 times (P < 0.05), which is conditioned by the activation of primary tumour growth. The number of lymphocytes as producers of vascular growth factor in primary tumour tissue under the influence of allogeneic mesenchymal stem cells of red bone marrow culture significantly increased by 1.47 and 1.52 times on Day 18 of the experiment compared to animals of the control group and placebo (P < 0.05), respectively. This promoted angiogenesis in the primary tumour node and metastasis through the circulatory system. After administration allogeneic mesenchymal cells of red bone marrow culture to mice, a larger volume of lung metastases was recorded, which was 41.52±7.9 mm3 compared to the values in the control and placebo groups, respectively, 17.94±6.59 and 16.43±5.32 mm3 . The morphological picture of the histological sections of the primary tumour of Lewis lung carcinoma confirms all the signs of qualitative and quantitative indicators of its progression. The findings obtained are of both theoretical and practical value for clinical veterinary medicine on the use of allogeneic bone marrow mesenchymal stem cells in tumour processes