1. The complement fixation test may be applied to the differentiation of the viruses of equine encephalomyelitis, lymphocytic choriomeningitis and St. Louis type of human encephalitis, respectively.

  2. The eastern, western and Moscow 2 strains of equine encephalomyelitis were clearly differentiated when using guinea pig brain antigens, but there was occasional cross fixation between the American strains when using mouse brain antigen. The Russian strain was quite distinct from either.

  3. There was no cross fixation between serums and antigens of the equine encephalomyelitic group and those of normal guinea pig and normal mouse or with the viruses of Borna disease, of lymphocytic choriomeningitis and of the St. Louis human encephalitis, respectively. Similarly there were no cross reactions between the antigens and serums of the 2 latter viruses nor between these and normal guinea pig brain antigens and serums, respectively.

  4. Complement fixing antibodies when once developed in hyperimmunized animals for the equine viruses may be retained for at least 3 months. The titer may decline but can be restored by virus injections. They may also be present in serums kept for at least 1 year and 10 months in the icebox.

  5. Neutralizing substances for the equine strains usually appeared somewhat earlier than the complement fixing antibodies.

  6. Antigens of the eastern, western and Moscow 2 strains of virus, respectively, gave positive fixation when passed through ultrafilters of 470 mu porosity. Potency was reduced by filtration through 200 mu membranes and entirely lost by those of 160 mu.

  7. Heating to 55°C. destroyed the antigenicity of the unfiltered western antigens and partially affected those of the eastern and Russian strains of equine encephalomyelitis. Heated filtrates were negative.

  8. The unfiltered mouse brain antigens of the American strains and those filtered through 470 mu gradocol membranes remained viable to mice, but not if heated or passed through finer filters. The Moscow 2 antigen remained viable only if untreated by heat or filtration.

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