St. Louis encephalitis (SLE) was first recognized as a nosological entity in 1933, when the single largest outbreak of the disease occurred in St. Louis, Mo. 1-3 The virus was recovered from mice and rhesus monkeys inoculated with human brain of affected cases. 4 , 5 Neutralization (N) tests, using the isolate and sera from recovered cases, confirmed that the etiologic agent had been identified and proved that the infection was distinct from other known etiologies of viral encephalitis. 5