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A formalin-inactivated hamster-kidney cell-culture (HKC) vaccine made with a highly attenuated strain of OCT-541 Japanese B encephalitis virus was tested in 41 human volunteers. Three doses of 1.0 ml were given intramuscularly in a primary series to most persons. By history, neutralization tests in vivo and in vitro, and a battery of group B arbovirus antigens used in HI and CF tests, the volunteers were divided into two groups—expected primary and secondary responders. Of primary responders, 92% had neutralization indices ranging from 3.0 to >6.0 logs after three injections. These rarely responded with hemagglutination-inhibition (HI) or complement-fixation (CF) antibodies. Secondary responders, as expected, boosted rapidly and broadly and reacted by HI and CF. Most primary responders still had good neutralizing antibody after 4½ and 8 months, and a fourth injection at 8 months boosted all to higher titers than ever before, all but one with indices between 4.0 and 7.0 logs, 50% of which were >6.0 logs. Many of these also gained HI and CF antibody. These responses are considered superior to those obtained with previous vaccines. The HKC vaccine, which produced minimal local and no systemic reactions, is theoretically considerably safer, being prepared from attenuated virus, containing no centralnervous-system tissue, and having a course of inactivation predictable and measured through its early phase.