Correlates of protection against influenza viruses have not been fully defined, but it is widely believed that protection against influenza can be conferred by serum hemagglutinin (HA) antibodies. The immune responses to injected influenza vaccines are routinely assessed by titrating serological HA antibodies. It is generally accepted that neutralizing and HA antibodies, as well as antibodies to neuraminidase, can be detected in serum 3–4 weeks post primary infection or vaccination. Serological assays commonly used to quantify antibodies specific for influenza viruses include hemagglutination inhibition (HI), single radial hemolysis (SRH), microneutralization (MN), ELISA and Western blot, of which, historically, HI and SRH are the most widely applied methods, the latter being increasingly replaced by MN. Each method used for antibody titration has different characteristics, and the validity index and specific use (seroepidemiology, serodiagnosis, response to vaccination, etc.) have to be considered while selecting the most suitable assay. Recently, ELISA tests have been improved, thanks to the elucidation of the structure of HA and the availability of this protein after recombinant expression. While the amount of data collected by conventional assays (HI and SRH) has permitted a fairly good optimization, serological measures are used to characterize the number of antibodies before and after vaccination. HI is the assay used most frequently for influenza antibody titration; however, it has low sensitivity in detecting responses to avian viruses in mammalian sera and alternative serological tests are needed. SRH utilizes a complement-mediated hemolysis reaction to measure the amount of antibody produced. This test appears to be as sensitive as the MN assay. HI and SRH assays are not functional tests for measuring immunity to influenza and suffer from several technical drawbacks. Improvements in these assays will be a further step in the preparation of new influenza vaccines, particularly for cell-derived products. Additional immunological assessments, such as cell-mediated immunity and the role of neuraminidase, need to be explored to give better insight into the overall effects of vaccination. [ABSTRACT FROM AUTHOR]