In 84 patients in the second or third stage of serologically proved Lyme borreliosis suffering from different forms of central and/or peripheral nervous system involvement the participation of autoimmunological mechanisms was investigated. The cellular hypersensitivity to encephalitogenic basic protein and to antigens from peripheral myelin was evaluated on the cytopherometer Opton according to the influence of macrophage slowing factor--liberated during the short-term incubation of the sensibilized lymphocytes with specific antigen--on the mobility of tanned sheep red blood cells in the electrical field. The frequency of positive findings in comparison with the results in other types of neuroinfections, examined previously, put the Lyme borreliosis on the first place. The explanation of this fact may be seen in the immunoregulatory disturbances, even though the changes of the total and active T lymphocytes were not significant. The knowledge of immunological changes is very important for the indication of immunomodulating procedures suitably complementing the treatment by antibiotics.