The trends in antigen expression and structural changes in the medulla of albino mice infected by West Nile virus (WNV) were studied in different periods of an infectious process, by using monoclonal antibodies to WNV. The maximum amount of immunoreactive material was found in the damaged neurons of dead animals, although WNV antigen expression in the perikaryonic cytoplasm of individual neurons was observed in all the experimental groups. Small amount of neuronal immunoreactive material and its significant levels in the glial cells of symptomatic animals are regarded as a manifestation of individual differences in the neuronal microelement.