In the spring and autumn of 1957, United States military personnel in the Far East were exposed to an influenza epidemic. Viral strains demonstrated an antigenic composition markedly different from those recovered in previous years.1 In the course of testing several hundred sera for the development of antibody to influenza virus, excellent results were obtained with the complement-fixation (CF) test in contrast to the standard hemagglutination-inhibition (HI) test, which often failed to demonstrate a diagnostic rise in antibody titer in sera positive by CF. Studies were undertaken in the autumn and winter of 1957-1958 to investigate the CF- and HI-antibody response to influenza in young susceptible adults in the Far East. Methods and Materials Antigens and Tests.—Complement Fixation: Far East-virus Strain A/JAP/307-57 was inoculated into 10-day-old embryonated eggs, and the uncentrifuged allantoic fluid was used as antigen in the complement-fixation test as described by Lennette,2 with the