1. A protection test for measuring serological protective properties against the encephalitis (St. Louis type) virus is described.

2. Normal non-contact sera and sera from persons supposed to have had no exposure to the disease do not protect against the virus. 82.5 per cent of sera from tested St. Louis encephalitis convalescents and at least 66 per cent of sera from tested persons thought to have had the disease do show protective properties.

3. The protective activity of sera is maintained for at least 2½ years after onset of the disease. In vitro aging of serum decreases its activity.

4. Protection tests indicate that the virus was present as early as 1932 in Paris, Illinois, spread through the north central states and reached New York in 1933, and was again active in the north central states in 1934.

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