Introduction. Cystic fibrosis is a common inherited disease that affects many organs and often leads to death. The treatment complexity is the resistance of its pathogenic strains to currently available drugs. The problem can be solved by the use of antibacterial herbal drugs. One of the sources of biol. active substances for antimicrobial herbal remedies can be common horse chestnut (Aesculus hippocastanum). Objective: To conduct a screening study of the antibacterial activity of aqueous alc. extracts from common horse chestnut flowers and buds, as well as chloroform extract and fraction of an individual substance of flavonoid nature, which are obtained from the plant buds. Material and methods. The investigation objects were aqueous alc. extracts from horse chestnut buds and flowers in 40, 70 and 96% ethanol, as well as alc. solutions of chloroform extracts of the flavonoid fraction from the plant buds. The min. inhibitory concentration (MIC) was estimated by the double serial broth dilution method. The clin. strains Burkholderia cenocepacia ST 208, Burkholderia multivorans, and Pseudomonas aeruginosa from patients with cystic fibrosis were used as test cultures. Results. The extracts from horse chestnut flowers were found to have a broad-spectrum antimicrobial activity and a pronounced bactericidal effect. The chloroform extract from the buds and the flavonoid fraction had lower activity. The extracts from horse chestnut buds showed a narrow-spectrum activity with sustained activity as antibacterial agents against Burkholderia cenocepacia ST 208 and Pseudomonas aeruginosa. Conclusion. The results of the investigation can be used as a rationale for including novel types of medicinal plant materials, as well as their based preparations in the State Pharmacopoeia of the Russian Federation.