FIELD: medicine； pharmaceuticals.SUBSTANCE: invention relates to pharmaceutical industry, specifically to use of a complex of water-soluble polysaccharides as an agent possessing immunomodulating activity, for inducing production of pro-inflammatory cytokines - interleukin-12, tumor necrosis factor-alpha by macrophages and interferon-gamma lymphocytes, inhibiting production of anti-inflammatory cytokines - interleukin-4 and interleukin-10 and stimulating an immune response caused by erythrocyte ram. Using a complex of water-soluble polysaccharides with molecular weights of 1053 kDa, 628 kDa and 35 kDa and their relative content of 61.24 wt. %； 5.37 wt. % and 33.39 wt. %, respectively, of duckweed separated from grass (Lemna polyrrhyza L.), as an agent exhibiting immunomodulatory activity, for inducing production of pro-inflammatory cytokines - interleukin-12, tumour necrosis factor-alpha by macrophages and interferon-gamma lymphocytes, inhibiting the production of anti-inflammatory cytokines - interleukin-4 and interleukin-10 and stimulating the immune response induced by erythrocytes of ram, wherein said polysaccharides are obtained by extraction for 3–4 hours of vegetal raw material, dried to air-dry state with moisture content less than 6 %, in a boiling water bath with purified water with addition of HCl pH = 4.0 in ratio of raw material: extractant - 1:20, filtration through the multilayer fabric filter, evaporation of filtrate to 1/5 of the initial volume on the rotary evaporator at a temperature of not more than 50 °C, settling for 24 h at temperature of 2–4 °C in triple volume of 96 % ethanol, filtration through a paper filter, dissolving the residue in purified water on a magnetic mixer for 3 hours at room temperature, removal of non-dissolved residue from fine particles of raw material and denatured protein by centrifugation for 30 minutes at 4000 rpm, dialysis through a semipermeable membrane with pore diameter of 15 kDa for 48 hours in 50-fold volume of
