The effect of quercetin on the functional and metabolic ac- tivity of neutrophils, DNA integrity, cell viability, and cell death in thymus and lymph node cells of mice under systemic endotoxemia induced by lipopolysaccharide (LPS) was inves- tigated. It was established that prophylactic administration of quercetin significantly attenuated the LPS-induced systemic inflammatory response, as well as cellular and organ dam- Вплив кверцетину на інтегральну цілісність ДНК і життєздатність імунокомпетентних клітин за умов ендотоксемії у мишей 25 age. The functional and metabolic activity of innate immune cells, particularly neutrophils, was reduced. In lymphocytes of the thymus and lymph nodes, the degree of DNA damage decreased: the percentage of cells with intact nuclei (class 0) approached control levels. At the same time, the percentage of nuclei with minimal (class 1), moderate (class 2), and severe (classes 3–4) DNA damage was significantly reduced. In addition, quercetin administration increased the viability of immunocompetent cells while reducing the proportion of cells with signs of necrosis and apoptosis. Thus, the use of quercetin in experimental endotoxemia contributed to the normalization of neutrophil functional activity, a reduction in single-strand DNA breaks in immunocompetent cells, and preservation of their viability. The observed cytoprotective effects of quercetin may serve as an experimental basis for further studies on its clinical application and for the development of new therapeutic approaches to the treatment of pathologies associated with Gram-negative bacterial components.