Preliminary experiments were initiated to test the attenuation for mice of chemically induced temperature-sensitive (E) mutants of a virulent strain of Eastern encephalitis (E) virus, and the potential of such mutants as live virus vaccines for mice. The reversion frequencies of eight mutants to temperature insensitivity were measured and the defects in their biosynthesis at the nonpermissive temperature were studied. All eight mutants were less virulent for mice, but the extent of avirulence varied with the mutant and route of injection. The mutants selected as potential vaccines protected mice against subsequent challenge at 10 to 21 days by the virulent virus of strain E. Neutralizing antibody activity was detected in almost all of the mutant-infected mice after 10 days, and was found in all infected mice at 21 days. Immunization by two doses of virus induced a very high protection against intracerebral challenge by virus strain E.


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