The experiments described in this paper were carried out with the Rockefeller Institute strain of albino mice and with the Eastern strain of the virus of equine encephalomyelitis.

1. The observation was confirmed that with increasing age of mice there occurred a decrease in susceptibility to intraperitoneal injection of active virus; also, the length of incubation period of those which succumbed increased with age.

2. The mice of various age groups which survived an intraperitoneal injection of active virus were indistinguishable in their antibody response.

3. Young mice, vaccinated with formalin-inactivated. virus when 2, 5, and 7 days old, gave an immune response to such a degree that they showed (a) measurable peritoneal immunity which increased with small increments of age, (b) no cerebral resistance, and (c) detectable amounts of neutralizing antibody in their sera which paralleled, though at a considerably lower level, their peritoneal resistance.

4. The peritoneal resistance induced as a result of vaccination was shown to be not local, but a general, systemic immunity, specific for the Eastern strain. Such a peritoneal resistance was demonstrable by the 4th day after beginning of vaccination of 10-days-old mice.

5. After intraperitoneal injection of active virus, large amounts of virus were recoverable from the blood of non-vaccinated young mice; none was found in the blood of vaccinated young mice; a minimal amount was detectable in the blood of non-vaccinated adult mice.

6. The bearing of age on the degree of immune response of which mice are capable and on their susceptibility to the virus has been discussed.

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