The earliest time at which serum antibody levels peak following administration of an influenza virus vaccine in elderly persons is not clearly defined. We compared the time intervals of 1 and 2 weeks after vaccination. A commercial trivalent vaccine containing the hemagglutinins of influenza viruses A/Texas/36/91 (H1N1), A/Johannesburg/33/94 (H3N2), and B/Harbin/7/94 was used. The hemagglutination inhibition (HAI) antibody titers at 1 week after vaccination were significantly lower than the HAI titers at 2 weeks postvaccination for all three vaccine components.