Objective. To investigate in vitro morphological and functional changes in the cells of microorganisms in the presence of lactic acid, to study its effect on the development of cultures of opportunistic bacteria (S. aureus, E. colli) and the fungi C. albicans and their ability to form a biofilm as compared with probiotics, and to explore the ability of probiotic lactobacilli to form biofilms (both in the presence and absence of lactic acid). Material and methods. The objects of the investigation were two test cultures of museum Lactobacillus strains; 5 Lactobacillus strains included in the probiotics ecofemin and lactoginal; 2 museum opportunistic bacteria strains, and 1 museum C. albicans strain. The investigators used different methods, including inoculation in the medium containing an agent, as described by O’Toole to explore biofilm formation; determination of the number of microorganisms in the biofilms according to the Koch method; and transmission and scanning electron microscopy. Results. Lactic acid, the active ingredient of femilex, has an antimicrobial effect, by changing the morphological features and functional status of the cells of opportunistic bacteria and fungi, inhibits the development of the cultures of E. coli by 4 times and S. aureus by 2.6 times and the microscopic fungi C. albicans by 2.4 times compared with controls, and also prevents the formation of biofilms. Lactic acid exhibits an average 2-fold greater antimicrobial activity than probiotics against opportunistic bacteria and fungi and at the same time stimulates the growth of lactobacilli and their ability to form biofilms. The combined effect of lactic acid and lactobacillus on the conditionally pathogenic microflora has been found. Conclusion. The present work is the first fundamental study of lactic acid as a main factor in maintaining the local biological homeostasis of the vagina.