The effect of Bifidobacterium strains: B. longum KN29.1 and KNA1, and B. animalis KSp4, on the level of bacterial antigen-specific IgA in serum blood of rats was studied using ELISA method. The experiment was carried out on 4 groups of young Wistar rats receiving daily for 14 days >10⁹ Bifidobacterium cells suspended in physiological saline (3 experimental groups) and bacteria-free physiological saline (control). Half the animals in each group were challenged orally with S. Enteritidis 458 as bacterial antigen, to which the level of the specific antibodies of the IgA class was determined by direct immunometric ELISA method. Briefly, the methodology comprised coating the microplates firstly with S. Enteritidis 458 in the amount of 2x10⁸ cells/100 μL/well, secondly with 100-fold diluted serum, conjugating with peroxidase-labelled goat anti-rat IgA conjugate (Nordic Immunology), and reading the absorbance at λ=450 nm. Application of the ELISA enabled comparison of the enhancement of the specific anti- S. Enteritidis IgA in serum blood of rats administered with probiotics. Those were significantly higher in all animal groups administered with bifidobacteria in comparison with the control group, both in non-infected and Salmonella-challenged animals. The obtained results indicate a direct stimulating effect of bifidobacteria on the immune system.