The possible immunological significance of the different kinds of cells participating in the inflammatory reaction in cerebrospinal infections is to be considered. Presence of plasma cells and occasional perivenous microglial seams in early stages of the neurotropic virus diseases must be regarded as immunological manifestations. The brainstem nuclei are sites of predilection both in entero- and in arbovirus diseases. The aetiologically different neurotropic virus diseases obtain their individual character from the involvement of certain neurones. Neural pathways are to be taken into consideration mainly for the propagation of enteroviruses, but may have a part also in the propagation of arboviruses. With massive invasion by highly cytopathogenic viruses inflammatory reaction may fail to appear in the immature nervous tissue. Some encephalitides, of both viral and non-viral aetiology, in which the prevalence of plasma cells in the infiltrations bears evidence of an intense immune process, may be interpreted as immunologically determined late diseases.