Mouse hepatitis virus (JHM strain) type 4 induces acute encephalitis followed by death in many strains of laboratory mice. Immunohistochemical study in vivo and analysis of mouse neuronal cells in vitro both indicate that the target cells in this infection is the neuron. Further, examination of several inbred mouse strains and neuronal cells from them shows that disease expression is controlled by a single autosomal gene action at the level of the neuronal cell. Susceptibility is dominant but not H-2 linked. However, cultured neuronal cells and macrophages from SJL/J mice, which are resistant to this infection, fail to make significant amounts of infectious virus after an appropriate viral inoculation. Apparently the defect is not at the level of the virus-cell receptor, because these cells, in part, express viral antigens.