Summary: The establishment and maintenance of a colony of mice free from mouse encephalomyelitis virus are described. Non-infected mice at all ages and by whatever route tested are more susceptible than latently infected animals to infection with the homologous strain. They are also more susceptible to heterologous strains of mouse encephalomyelitis virus. The infectivity of mouse encephalomyelitis is influenced by maternal passive immunity, by acquired humoral immunity, and by the resistance that develops with age. Inhibition of mouse encephalomyelitis virus by the feces of infected mice is confirmed.