After intranasal inoculation with a neurotropic mouse hepatitis virus, JHM strain, suckling mice less than 2 weeks of age died in 4 or 5 days postinoculation, while those 3 weeks or more of age survived. In the brain lesions of mice 8 days or less of age, there appeared two different types of multinucleated giant cells. One of them which was irregular in shape with many virus particles, appeared mostly in the subependymal or perivascular areas after 72 h postinoculation and it seemed to have resulted from a fusion of two or more infected macrophages. The other type of giant cells which was oval in shape with a small number of virus particles, appeared within degenerative lesions at 96 h postinoculation. It consisted of two morphologically different parts and seemed to be a complex of infected neurons and reacting macrophages. In the cerebral lesions of mice 2 to 4 weeks of age, only the former type of giant cells appeared.