1. Animals injected with emulsions of monkey brain with adjuvants show a complex pattern of antibody response as determined by complement fixation tests.
2. Organ-specific complement-fixing antibodies to constituents of brain tissue may be formed which fix complement with brain tissues of various animal species but fail to react with other organs or with rabbit placenta.
3. Antibodies may be formed to some constituent of brain other than nervous tissue. It would seem that these can be detected by the strong complement fixation given with rabbit placenta.
4. Sera from individual animals may contain antibodies to the brain or placenta constituents, to both, or to neither. Occasional individual sera show unique patterns of antibody response as determined with various additional antigens such as fetal brain, posterior pituitary, or peripheral nerves.
5. No evidence of any etiological relationship between the development of encephalomyelitis and the complement-fixing antibodies to brain demonstrable in the sera could be found. The complement-fixing antibody to the placental constituent was unrelated to the encephalomyelitis.