Edelman, R. (Dept. of Virology, U.S. Army Medical Component, SEATO, APO San Francisco, Calif. 96346), A. Nisalak, A. Pariyanonda, S. Udomsakdi, D. 0. Johnsen. Immunoglobulin response and viremia in dengue-vaccinated gibbons repeatedly challenged with Japanese encephalitis virus. Am J Epidemiol 97: 208–218, 1973.—The response of primates to serial group B arbovirus infections was investigated by inoculating gibbons with live Japanese encephalitis virus (JEV) 17 to 21 months after infection with multiple dengue virus serotypes. Five of 8 gibbons developed detectable JE viremia, and all 8 animals had high-titered cross-reactive serum antibody titer rises to group B antigens, indicative of JEV replication in these dengue-vaccinated gibbons. The immunoglobulin response was characterized by early production of high.titered cross-reactive IgG antibody, and later production of low-titered IgM antibody reacting monospecifically with JEV. Thirteen months later 7 of these gibbons were re-inoculated with JEV. In contrast to the first JEV challenge, no viremia was detected and the serologic response, including IgM antibody production, was partially or completely aborted. This study indicates that isolated serum IgM antibody, reacting monospecifically with JEV antigen, may provide a useful and specific serologic test for first JEV infections in hosts previously sensitized with dengue. In addition, the results suggest that prior infection with JEV, but not dengue, can protect gibbons against JEV inoculated more than one year later.