Guinea pigs can be immunized against lymphocytic choriomeningitis by 2 or 3 injections with formolized vaccines prepared from a variety of infected guinea pig tissues. Vaccines prepared from the consolidated areas of diseased lungs gave the best results. The immunity produced was partial in the majority of the cases, in that the vaccinated animals as a rule showed fever after the test of immunity and virus was present in the circulation during the febrile period.
Vaccines prepared from infected mouse tissue had no, or very little, immunizing power for guinea pigs, even when prior to formolization they contained at least as much virus as guinea pig tissue vaccines. This failure to immunize appears to be due to the interference by heterologous antigens, since the immunity induced by homologous vaccines was often inhibited when formolized normal mouse tissue suspensions treated in the same manner as the guinea pig tissue vaccines were added to the latter before inoculation. The inhibitory effect of the heterologous tissue was less marked when it was not mixed with the vaccine but injected simultaneously on the opposite side of the body.
The immunizing power of homologous vaccines did not parallel their virus content prior to formolization.
A high degree of immunity, characterized by protective antibodies in the serum, was produced in some guinea pigs by prolonged treatment with large doses of homologous vaccine, while sera of guinea pigs vaccinated in the ordinary manner contained no detectable neutralizing antibodies. It is possible, therefore, that the immunity produced by inactive virus differs only quantitatively from that induced by an infection.