Serological verification of ixodid tick-borne borrelioses (TBB) is carried out in Russia by using IFA with the standard antigen prepared from the strain Ip-21 B. afzelii by the Laboratory of Infection Vectors, Gamaleya Research Institute of Epidemiology and Microbiology, Russian Academy of Medical Sciences. This study is under way at approximately 50 research and practical institutions that are situated in 30 large administrative territories of the country. The nIFA data on 371 sera from 214 patients with TBB and 229 sera from controls who had other infections have been analyzed. Changes in the proportion antibody sera, mean geometrical and cut-off antibody titers in patients with TBB were compared with those in the controls. The values of seroconversion were analyzed in relation to the time lasted after the onset, the first serum collection, and an interval of reexamination. The specific results of tested of the coded sera were obtained during the international interlaboratory blind experiment. It is concluded that nIFA yields results that are suitable for TBB verification.