FIELD : medicine; pharmaceuticals.SUBSTANCE : invention relates to pharmaceutical industry, namely to use of composition as antiviral and/or antibacterial agent in individuals undergoing anti-tumour chemotherapeutic treatment, treating acquired immunodeficiency syndrome and treating leukaemia. Use of the composition containing a mixture consisting of : a strain of bacteria belonging to type Lactobacillus reuteri, identified as Lactobacillus reuteri LRE03, with depositary number DSM 23879 and/or bacterial strain belonging to type Lactobacillus salivarius, identified as Lactobacillus salivarius LS06, as an antiviral and/or antibacterial agent in individuals undergoing anti-tumour chemotherapeutic treatment, treatment of acquired immunodeficiency syndrome and treatment of leukaemia.EFFECT : composition described above stimulates production of anti-inflammatory cytokines (Th1) of interferon INF-gamma, wherein said cytokines exhibit pronounced antiviral and/or antibacterial activity, stimulate production of dendritic cells, wherein said dendritic cells also exhibit manifested antiviral and/or antibacterial activity.10 cl, 2 tbl, 6 dwg