1. The susceptibility of mice of different ages to intramuscular, intraperitoneal, and oral administration of EMC virus, and to intracerebral and intramuscular injection of both the Texas and the Connecticut-5 strains of Coxsackie virus have been studied. In all cases susceptibility decreases with age. The development of resistance to a given virus may vary in rate and extent according to the route by which the virus is given.
2. The multiplication of small amounts of EMC virus in muscle, and of the Texas and Connecticut-5 strains of Coxsackie virus in both muscle and brain, has been studied in mice of different ages. A very high degree of multiplication of all three viruses was observed in both muscle and brain of young mice, but no significant multiplication of the EMC virus in older muscle could be detected with these small inocula, nor of either of the Coxsackie strains in older muscle or brain.
3. Multiplication of EMC virus in young mice was shown to take place within the muscle itself. Haemagglutination tests showed that less haemagglutinin, as well as less infective virus, was produced in the muscles of older mice following the injection of small amounts of virus.
4. Multiplication of EMC and GDVII viruses could be demonstrated even in older mice if a sufficiently large amount of virus was inoculated initially.
5. The rapid initial disappearance of infectivity from the site of inoculation was shown to be mainly due to dispersal of inoculated virus around the body.
6. The simplest hypothesis which will accommodate the observed data on virus multiplication and on the development of resistance with age appears to be that the accessibility of the host cell to virus decreases with age. We found no evidence that the ability of the host cell to support virus multiplication declines with age.
This work was done in the Department of Zoology, Oxford University. One of us was in receipt of a Medical Research Council training grant. The expenses of the research were met by a grant from the Nuffield Foundation, to whom we should like to express our gratitude.