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SUMMARY Strains of mice differ greatly in their susceptibility to mousepox. The highly resistant strain C57 Bl has been compared with a susceptible strain of mice with regard to virus growth and antibody production. Differences in virus growth in the two strains of mice appeared at a certain stage of the infection, which was the time at which neutralizing antibody was demonstrable in the blood of resistant mice. Neutralizing antibodies, active immunity and hypersensitivity were shown to appear at least a day earlier in the resistant than in the susceptible mice. The highest degree of resistance was found in young adult C57 Bl mice (2–3 months). Resistance was very low in new‐born mice and very old mice (8–12), although death times were still delayed compared with those of susceptible mice of the same ages. The resistance of C57 Bl mice to the lethal effects of infection with ectromelia virus appears to be due to a more effective immune response than that shown by mice of susceptible strains.

SUMMARY The response of mice to ectromelia virus depends on the route of injection and the differences between C57 Bl and a susceptible strain of mice have been studied using different routes of injection. The difference in susceptibility between C57 Bl and stock mice appears earliest following the intravenous injection of ectromelia virus, and following splenectomy this difference is abolished. The strain difference is most pronounced when the virus is injected into the footpad. It is reduced to a minimum after intracerebral injection, probably because not enough antibody reaches the brain tissue. All resistance is abolished after intraperitoneal injection and for this no explanation is advanced. Following the intranasal instillation of virus, C57 Bl mice are less resistant because they die of the pneumonia which follows a vigorous local response in the lungs. They succumb to doses of virus which are not lethal by the footpad route, and with larger doses they even die earlier than stock mice. The difference in susceptibility following different routes of inoculation can be explained in terms of the difference in effectiveness of the antibody response and it has been shown that resistance is inherited as a single, autosomal, dominant factor.