1. The infectious agent from fatal cases of St. Louis and Kansas City encephalitis passes Seitz pads in high dilution without appreciable loss of infectivity and traverses collodion membranes with an average pore size of 66 mµ or greater.

2. It is highly infectious for mice by the intracerebral and intranasal routes, but practically innocuous by the subcutaneous and intraperitoneal routes.

3. Certain strains of mice are more susceptible than others.

4. The agent administered to mice intranasally causes tremors and convulsions after a 6 to 7 day incubation period, followed by prostration and death in 8 to 10 days. Lesions are demonstrable in the olfactory bulbs 3 days after infection, in the piriform lobe after 4 days, and in Ammon's horn after 5 days.

5. In Macacus rhesus monkeys, the agent provokes a mild, non-fatal reaction and the development of specific neutralizing bodies. On passage in monkeys, the virus becomes progressively weaker.

6. In rabbits, guinea pigs, rats, and sheep the agent is apparently without effect.

7. All available strains of the agent proved alike in effects in animals and in immunological response.

8. The available data enable one to conclude that the agent is a filtrable virus differing from those studied heretofore.

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