1) — Two strains of virus which produce myeloencephalitis were isolated from two white swiss mice, from breeding, spontaneously infected, among 7000 mice examined; another strain was obtained by trituration and filtration of the intestines of normal mice. 2) — There were made separately ten serial passages in young and adult mice by intracerebral route. By crossed immunological test it was verified that the three strains were identical, and so only one was continued. 3) — The infective power grows with the number of passages: the average incubation period diminishes and the mortality ratio increases highly. 4) — The spontaneous and experimental disease is described. The infec¬tion is more commonly among young mice. The average incubation period varies from 5 to 30 days. Sometimes it is possible to observe a prodromal period: weakness, smaller activity, difficulty of locomotion; generally there appears a fiacid paralysis without previous symptoms, most commonly on the hind legs. Clinical forms: superacute, acute and chronic. 5) — In normal mice virus can be demonstrated in the feces and also in the walls of the intestine. The virus appears to be a common inhabitant of the alimentary tract and only occasional invader of the central nervous system, in other words, the encephalomyelitis is primarily an alimentary tract disease in which invasion of the nervous system is only an occasional incident. 6) — The virus is filtrable by Chamberland L3 and L5, Berkefeld V, N and W and Seitz EK; the suspension is as active as before filtration. It will keep well at frigo at least during 60 days in glycerine at 50%. Brain and cord suspensions when heated in hot bath at 56°C by 30 minutes, lost their activity. 7) — The title has varied between 4.000 to 20.000 M. L. D. 8) — Infection was obtained by intracerebral inoculation, nasal instilla¬tion and, with less regularity, by intraperitoneal inoculation; the gastric route was always negative. Very young mice are more susceptible than adult ones. 9) — Virus was always isolated up to 90 days post-inoculation from brain and spinal cord of mice with paralysis. Animals inoculated through in¬tracerebral route, which remain apparently normal, keep the virus in the brain at least during 30 days. 10) – Is was not possible to find virus in the lungs, livers, apleen, kindneys and blood of mice inoculated intracerebrally. 11) – Mice which received virus by intercerebral route and did not show symptoms of infection, generally were immune to another administrations of virus. The convalescent mice sera showed positive protection test. 12) – Young rhesus, rabbit, guinea pigs and rats, inoculated in intracelebrally with relatively large doses of encephalomyelitis virus failed to show symptoms referable to the virus.