The purpose of the investigation was to identify the role of systemic endotoxinemia in pathogenesis of gestosis (preeclampsia) depending on the degree of severity of the clinical manifestations. Complex clinical and laboratory examination of 134 pregnant women was undertaken, 74 of them were diagnosed with gestosis (a study group). They were divided into 3 subgroups: 27, 23 and 24 pregnant women with mild, moderate and severe gestosis respectively. The control group consisted of 60 pregnant women with physiological course of gestation. The levels of bacterial lypopolysaccharide (LPS), LPS-binding protein and IgG antibodies to core-region of LPS, cytokines (TNF-α, IL-1β, IL-2, IL-4, IL-6, IL-8, IL-10, IFN-γ), IgA, IgM, IgG, C -reactive protein were determined in plasma. It was shown the increase in LPS, IgG titers to the core-region of LPS and LPS-binding protein in the blood plasma of pregnant women with gestosis. Changes in cytokine balance at gestosis were characterized by increased levels of IL-1β, IL-8, TNF-α и IL-10 and reduced concentration of IL-2 and IFN-γ at the same time. The differences in the level of systemic endotoxinemia depending on the clinical manifestations of preeclampsia were established. The results of the study indicate the involvement of LPS gram-negative bacteria in the maintenance of immune reactions at gestosis.