The present knowledge of research makes it possible to use a whole range of stabilized micro-organisms and their metabolites for the production of preparations known as probiotics. The mechanism of the effect of probiotics, although it is not fully elucidated, is closely connected with the properties of the production strains. When selecting them, it is necessary to respect the origin of the strain used, its ability to adhere to the epithelial cells of the gut and to produce the inhibitory substances, the ability to survive and grow in the respective ecological units. The strain should be genetically stable, it should have good growth properties in vitro and in vivo, to maintain its high viability at processing, lyophilization and storage. Lactobacilli belong to the micro-organisms most frequently used to prepare the probiotics. The statement that lactobacilli inhibit the colonization of pathogenic bacteria upon the intestinal epithelium has been confirmed by many experiments on experimental germ-free and/or gnotobiotic animals. This inhibitory process known as "competitive exclusion" can be explained by the competition for the adherence sites on the intestinal mucosa between pathogens and lactobacilli and by the production of inhibitory substances. The host specificity as well as different degree of the expression of adherent phenotype, which is conditioned, in addition to the effect of the external environment also by the presence of plasmids of adherence, present the important property of the adherence of lactobacilli. The host specificity of the lactobacilli is closely connected with the presence of the specific molecules of receptors on the host cells which can be distinguished by means of specific molecules of the bacterial cells. The competition for the nutrients to be found on the intestinal epithelium, which present the growth substrates for both the probiotic strains and the pathogens, may present an important factor influencing the colonization of lactobacilli. The inhibitory components of the lactobacilli comprise the production of bacteriocins, toxic metabolites of the oxygen and organic acids. The inhibitory spectrum of the bacteriocins is different; the overwhelming majority of them inhibits G+ bacteria within the genus or taxonomically close to the genus of Lactobacillus, others such as bulgaricin, acidolin, laktocidin, acidophilin and non-protein substance reuterin are active against a wide spectrum of G+, G- bacteria, yeasts and fungi. Of toxic oxygen metabolites, the production of hydrogen peroxide is of importance. The hydrogen peroxide along with lactoperoxidase-thiocyanate milk system exerts bacteriocid effects on most pathogens. The production of organic acids (lactic, acetic) by lactobacilli is considered to be the primary regulator of the gut microbial activity in animals. The lactic acid as a product of the intestinal fermentation is further metabolized by lactate utilizing bacteria to VFA, thus the environment of the digestive tract is acidified and the acidosensitive pathogens cannot exert their effect. When pH of the environment falls, the concentration of molecular (non-dissociated) form of weak organic acids (acetic acid, VFA) elevates. The above acids diffuse through the cell membrane into a cell, thereby exhibiting higher inhibitory capacity. In case the mechanism of the effect of probiotics is fully elucidated, the criteria of the selection could be more precisely defined and thereby also the efficiency of probiotics would be improved.