A follow-up examination of blood sera and cerebrospinal fluid was carried out in 413 patients with various neuroinfections and related diseases. The modern immunological methods were employed: the complement fixation test, the fluorescent antibody test as well as immuno-enzymic and radioimmunoassays. It was established that 8.5% of serous meningitides and 12% of encephalitides were induced by lymphocytic choriomeningitis (LCM) virus. The verified diseases were subjected to a clinical analysis. It is emphasized that the immunological examination of the cerebrospinal fluid in patients with LCM infection contributes to a more detailed study of the pathogenesis of the disease.