Lyme borreliosis is characterized by rich symptomatology which in a very high percentage resembles early stages of connective tissue diseases. Some patients after having contactes a physician are referred to a rheumatologist. The rheumatologist may order a test for “marker” antibody and a test for presence of anti-bacterial antibodies (anti-Borrelia burgdorferi). Frequency of positive results of anti- B. burgdorferi is rather low (14.7%), and because of that, there is a problem of credibility of the above-mentioned test. Seronegativity, as to the presence of specific antibodies to B. burgdorferi, concerns about 40% of sera of the patients with borreliosis, including patients with confirmed incidents of the tick bite and appearance of the pathognomonic manifestation (migrating erythema). Despite many bacteriological, serological or molecular (PCR) tests available, diagnosis of borreliosis causes difficulties and may lead to false negative or positive results. Authors of this work focused on cases of patients with sera seronegative for antibodies to B. burgdorferi , where free antibodies have been probably bound to bacterial antigens, forming circulating immune complexes (CIC). The goal of the researches was to analyse composition of CIC for the presence of antibodies to B. burgdorferi in sera of patients difficult to diagnose, where the clinical syndromes suggested the infection with B. burgdorferi (Fig. 1-7). The researches covered sera of a group of 54 patients with suspected borreliosis. A new methodological approach to analysis of the antibodies specific to B. burgdorferi was applied. This method is based on precipitation and dissociation of CIC, and estimation in prepared γ fraction of antibodies specific to B. burgdorferi In 41 sera negative in the ELISA test, a γ fraction from PEG sediment was obtained and in prepared γ fraction antibodies against B. burgdorferi antigens were estimated. In 82% of sera, such antibodies were found, mainly in the IgG class. The confirmatory Western-blot test was applied and in 50% of tested γ fraction, the presence of specific anti- B. burgdorferi antibodies was confirmed, in 22% – equivocal results were obtained and in the remaining 28% – antibodies to antigen p41 were found. Authors conclude that this new methodological approach that they developed, after simplification necessary for routine diagnosis, may be useful in sero-diagnosis of borreliosis and will diminish the percentage of false negative results in a significant way.