The antibody response to influenza vaccination has been variably reported to be decreased in elderly individuals. To determine the effect of ageing alone on this antibody response, a group of carefully-screened healthy elderly subjects were compared with young adult controls. Antibody titres for several strains of influenza were measured before and after vaccination with whole-virus (WVV) and split-virus influenza vaccines (SVV) in two successive years. In general, the antibody response to WVV was greater than the response to SVV. Both groups showed a similar response to the H3N2 strain but the elderly group showed a lower response to the H1N1 and B strains of virus contained in the vaccine. Antibodies to older strains of influenza A but not B were stimulated by vaccination with SVV. In the elderly group, the response to older viral strains was relatively increased compared with newer strains. In contrast, the young control group had better antibody responses to the newer than to the older strains of influenza tested. Reductions in the antibody response to influenza vaccination may, therefore, be related to the phenomenon of original antigenic sin and the cohort effect of exposure to H1N1 during childhood in the elderly group studied. The increased immunogenicity of WVV must be considered in light of the current wide use of SVV in the elderly.