Four vaccines are available in the United Kingdom against influenza virus. All are subunit vaccines, defined as either split-virion or purified surface antigen vaccine; there are two of each distinct type available. Both vaccine types are less reactogenic than whole inactivated virus, with antigenicity induced by viral surface glycoproteins. Here, each of the four vaccines has been characterized by electron microscopy and SDS-PAGE analysis, giving a unique vaccine profile. Three vaccines contain internal viral nucleoprotein which, in the presence of residual haemagglutinin, may induce an influenza A virus cross-reactive cytotoxic T-cell response and thus be of value to vaccine efficacy. Residual lipid was present in three vaccines and recent evidence suggests that pyrogenicity is correlated with the presence of viral lipid with clusters of surface glycoproteins. By a combination of electron microscopic evidence and biochemical characterization, it has been possible to resolve compositional differences, not only between vaccine type, but also between each individual currently available vaccine. Hence, there is the possibility that the morphological differences characterized here may be contributory to potential reactogenic effects subsequent to vaccination.