The purpose of the study was to determine the antagonistic effect of Lactobacillus strains, isolated from saliva of healthy people on clinical strains of H. pylori. Two kinds of Lactobacillus strains were used in the tests i.e. strains producing hydrogen peroxide as well as strains not producing the metabolite. Clinical isolates of H. pylori were taken from the patients with confirmed duodenal and gastric ulcers. The evaluation of antagonistic effect between microorganisms was performed by using agar slab method. It was observed that Lactobacillus acidophilus had an antagonistic effect on growth of all strains of H. pylori tested. However, the particular Lactobacillus strains differed with respect to their inhibitory properties. Hydrogen peroxide did not have any significant impact on the inhibition of the growth of Helicobacter pylori.