In a trial of influenza virus vaccines, 3,103 students (13.6% of a campus population) were given a standard dose of vaccine (400 chick-cell agglutination [CCA] units of influenza A and 300 CCA units of influenza B) in October or December 1971. Fifty-eight percent of the vaccinees received a tri-(n-butyl) phosphate (TNBP) split virus vaccine; 42% received inactivated whole virus vaccine. In January, a discrete influenza outbreak occurred. Influenza A/HK virus (H3N2) was recovered as the predominant infecting agent. In a three-week period, 168 students sought medical assistance for febrile influenza. Only eight (4.8%) of these were vaccinees, representing a 69% reduction from the attack rate in nonvaccinees. The protection derived from split virus and whole virus vaccine was similar.