Two agents isolated in 1953 from patients in epidemics of meningo-encephalitis in Slovenia and Austria were examined in neutralization tests at the Army Medical Service Graduate School in Washington, D.C., USA, and were found to be indistinguishable under the conditions of test from the agents called Russian spring-summer encephalitis virus, Czech encephalitis virus, or louping-ill virus, which are believed at the present time to be strains of the same agent.