The antagonistic activity of 4 strains of bifidobacteria (B. adolescentis 2 F1, B. longum Z4, B. breve R2 and B. bifidum G1), isolated from the vagina of healthy females of the reproductive age, with respect to Escherichia coli, Klebsiella ozaenae, Staphylococcus aureus, Enterococcus faecalis, Pseudomonas aeruginosa and Gardnerella vaginalis were studied in vitro and in vivo. The in vitro experiments revealed that all above-mentioned bifidobacteria were capable of inhibiting the growth of all indicator bacterial strains. Still of all the bifidobacteria under study had different levels of activity. B. adolescentis strain 2 F1 exhibited the highest inhibiting activity in vitro. In contrast to in vitro experiments, in vivo experiments with B. breve R2 demonstrated its high antagonistic activity with respect to E. coli. The data thus obtained indicate that in the study of antagonistic activity the use of the in vivo model as also expedient, for it is mainly in vivo that probiotic preparations show their activity.