The objective of the research study was to determine the survival of strains of Lactobacillus, isolated from food, under the conditions of a gastrointestinal (GI) tract model. The research material constituted two strains of Lactobacillus isolated from oscypek and bundz cheeses, and a probiotic strain of Lactobacillus plantarum 299v. It was proved that the addition of lysozyme and pepsin to gastric juice and of pancreatin to intestinal juice had no significant effect on the survival of Lactobacillus. There is a relationship between the viability of cells and the incubation time of bacteria strains studied. The 5 h lasting incubation of strains in the simulated GI tract model caused the count of bacteria to decrease by 2 ÷ 3 logarithmic orders. The pH value of gastric juice had the greatest effect on the survival of the bacteria studied. Furthermore, it was found that the addition of a food medium in the form of UHT milk increased the tolerance of the bacteria studied to the gastric juice simulant and had a protective effect. Also, it was found that the Lactobacillus strains analyzed, when treated by intestinal juice, tolerated a high physiological concentration of bile salts. The results obtained can be applied to optimize discriminant methods in in vitro studies on the ability of potentially probiotic bacteria to survive under stress conditions of human GI tract.