Abstract
1. Susceptible mice injected subcutaneously or intraperitoneally with 15,000 intracerebral lethal doses of St. Louis encephalitis virus develop an immunity in 4 to 7 days to 1,000 to 1,000,000 lethal doses given either intracerebrally or intranasally. 2. This immunity persists 4 to 6 weeks, then decreases gradually and disappears after 8 to 12 weeks. 3. More than 1,000 intracerebral doses of virus given as a vaccine do not materially increase the amount or duration of the immunity; less than 1,000 doses give little or no immunity. 4. Test virus injected intracerebrally into immunized mice induces few lesions and is rapidly destroyed; instilled intranasally, it rarely reaches the olfactory lobes or brain. 5. While immunity is maximum, circulating neutralizing antibodies are not detectable. Moreover, the immunity is not affected by endothelial cell blockade or by splenectomy. 6. A few moments after the immunizing virus is given, it can be recovered from the blood in relatively high concentration. After 24 hours, the blood no longer contains demonstrable virus nor do any organs thus far tested except the spleen. The brain and cord remain entirely normal. The spleen, however, becomes enlarged and harbors virus for as long as 30 days.