Plasma samples from 1,459 bats collected in Japan during 1963–1965 were assayed for serologic evidence of infection with JBE virus by an in vitro plaque-reduction method. Neutralizing antibodies were demonstrated in 8% of the specimens of plasma from bats netted in Honshu and Kyushu, Japan, whereas only 3% of the specimens from bats netted in Hokkaido, the northernmost part of Japan, were positive. Bats with neutralizing antibodies against JBE virus were found with about equal frequency in groups collected during each of the four seasons of the year. Serologic evidence of infection with JBE virus could not be detected by the HI test. Antibodies against JBE virus were also demonstrated in 7% of 275 specimens of plasma tested by the in vivo neutralization test in weanling mice. Similarities between certain characteristics of the immune response of bats to natural and experimental infection with JBE virus are discussed.