Abstract
1. St. Louis encephalitis virus injected intracerebrally or intraperitoneally in maximum doses in resistant mice is distributed and is effective in a manner generally similar to that in susceptible mice. The minimum infecting dose is at least 1,000 times greater in resistant than susceptible mice and virus injected in the brain tends to remain at a relatively low titre, persist for a few days, and then disappear. 2. Virus dropped in the nares is demonstrable and progresses in the brains of resistant mice as in susceptible mice, but does not increase in titre beyond the 5th day, does not bring about fatal encephalitis, and persists for at least 4 weeks. 3. Lesions in the brains of resistant mice following nasal instillation of virus do not appear until the 8th day, reach a maximum at 40 days, and are still present, though resolving, at 3 months. The changes resemble those seen in the human disease and in other unnamed forms of encephalitis. 4. The quantity of virus drops 1,000-fold when recovered from resistant mice and becomes non-infective by the nasal route. Passage in susceptible mice promptly restores its full titre.