1. As mice grow older they acquire a resistance to peripheral inoculation with the Indiana and New Jersey strains of vesicular stomatitis virus and to some extent also to Western equine encephalomyelitis virus, but little or none to the Eastern strain.
2. While some mice may become resistant as early as the 30th day of life, others may still be susceptible at 1 year of age.
3. This resistance is readily demonstrable when the inoculations are made by intranasal, subcutaneous, intramuscular, intraperitoneal, and intravenous routes, but not when the virus is injected directly into the brain.
4. The resistance is not related to previous exposure to infection or to the presence of specific or nonspecific antiviral bodies in the blood.
5. No difference in susceptibility to peripheral inoculation was found in young and old guinea pigs to pseudorabies virus, and in relatively young and old Macacus rhesus monkeys to poliomyelitis virus.