1. French neurotropic, Asibi and vaccine (17D) virus, infect young mice (21 days) when instilled in the nostrils, killing them with symptoms of encephalitis or developing immunity. 2. French neurotropic virus and Asibi (and perhaps vaccine virus), when instilled in the ear of young mice, may infect them producing development of immunity. 3. After instilation in the scarified cornea of young mice, French neurotropic virus, Asibi and perhaps vaccine virus, may produce infection. The cornea being, however, whole it has not been able to infect mice. 4. French neurotropic virus, Asibi, vaccine, Asib Egg I and Egg III introduced into the stomach, may infect young mice, causing e fatal encephalitis or rendering them immune. 5. It is possible to infect mice 0-6 days old by simple deposition of virus or rubbing the suspension on the whole and scarified skin.