1. The technique of the intracerebral protection test in mice is described in detail, particular attention being paid to the preparation and maintenance of an antigen of constant titre to permit of quantitative comparison of the results of different tests.
2. The specificity of neutralization in vitro is illustrated.
3. The delayed .appearance of virucidal antibodies in the serum of horses following immunization is established.
4. The difference in rate of production of demonstrable antibodies following immunization by the neurotropic virus method and the serum-virus method is indicated.
5. The relation between anti.-body content of the serum and total immunity is discussed.
6. The plurality of strains of horsesickness virus is demonstrated.
7. The result of an experiment to indicate the possible antigenic structure of the virus is discussed.
8. The possibility of the production of high titre hyperimmune serum is indicated.
9. The results obtained are discussed in the light of previous work on the viscerotropic virus of horses and mules.
10. Failure to devise an in vitro test to replace the biological test at present essential for the completion of neutralization experiments in recorded.