1. Sera of animals immunized against Japanese B encephalitis, Venezuelan equine encephalomyelitis, and Western equine encephalomyelitis viruses were fractionated by electrophoresis.
2. Electrophoretic patterns of rabbit sera before and after immunization against Japanese B virus showed no consistent change traceable to antibody formation.
3. To determine the antibody content, the electrophoretic fractions of the respective sera were mixed in varying dilutions with infected mouse brain suspensions, and the neutralizing titers of the fractions were compared.
4. In all instances serum fractions containing γ-globulin were protective, whereas in no case did serum albumin show any virus-neutralizing activity. The Japanese B encephalitis antibody appeared to be associated entirely with the γ-globulin. The Venezuelan and Western equine encephalomyelitis antibodies were associated with the ß- and γ-globulins and probably possessed an average electrophoretic mobility between that of ß- and γ-globulins.
5. Normal rabbit serum similarly separated electrophoretically showed no neutralizing properties.
6. Chickens, whose electrophoretic serum pattern is markedly different from that of rabbits, were also immunized against the Japanese B encephalitis virus. Their antisera were electrophoretically fractionated and similarly subjected to neutralization tests. The specific neutralizing capacity of chicken serum was considerably lower than that of rabbit serum and no neutralizing activity was found in the fractions containing the faster moving components. The antibody appeared to be associated with component 4 which had a mobility of approximately 2.3 x 10–5 cm.2/volt/sec.