Addition of protamine sulfate to chick-embryo or mouse-brain suspensions of certain viruses precipitated much of the tissue material while leaving most of the virus behind in a clear supernatant fluid. The excess protamine remaining in the latter fluid could then be precipitated out with heparin. The following agents remained in the supernatant after protamine treatment: Colorado tick fever, equine encephalomyelitis (western type), encephalomyo-carditis, Japanese encephalitis, poliomyelitis (Lansing strain), Russian Spring-Summer encephalitis, St. Louis encephalitis, West Nile virus. Those precipitated along with the tissues by the action of protamine were; Herpes, lymphocytic choriomenin-gitis, murine encephalomyelitis (GD-V11 strain), rabies, and vaccinia. The total N content of a prepn. of Japanese encephalitis virus purified by protamine precipitation and ultracentrifugation was only 10% of that of a similar prepn. purified by ultracentrifugation alone. Protamine-purified Japanese encephalitis vaccines (mouse brain type) have the same degree of immunogenic potency as crude mouse brain suspension when assayed in mice and human volunteers.