A special epidemiological experiment to prevent borreliosis in persons bitten by infected ticks was performed in 1992-1994 in the Russia's Perm region where Borrelia garinii and B.afzelii circulate, and Ixodes persulcatus tick is the sole vector transmitting these pathogens to human beings. Adult ticks were removed from the bodies of persons who had referred to health facilities for first aid. Vital preparations were made from the material obtained from the gut of each tick and examined microscopically (up to 250 microscopic fields per preparation). The patients bitten by infected ticks were divided into experimental and control groups and kept under special medical and serological control for 4-5 months. The patients of the experimental group received doxycycline (100 mg twice daily) for 3-5 days after ticks had bitten. Borreliosis was diagnosed by a combination of clinical and serological data. The control group consisted of 97 patients who took no antibiotics after ticks biting and 12 of them contracted borreliosis. In 823 cases Borrelia were not revealed while microscopically analyzing the ticks removed from the patients' bodies; in this group six patients contracted borreliosis. The morbidity rate (per 100 patients) in the experimental group was 1.1, i.e. 11 times lower than that in control group. Among the patients bitten by infected ticks and untreated with antibiotics, this index was 17.6 times higher than in the group bitten by ticks in which Borrelia were not found. There is no absolute probability of detecting the pathogen during a direct microscopic analysis of the preparation made from the tick removed from the body of a bitten patient. However, this rapid identification of Borrelia, followed by short-term antibiotic treatment for microbiological evidence is an effective tool for preventing patients from contracting borrelioses.