At present influenza vaccines are standardized on their haemagglutinin content only. Recently it has been shown that both neuraminidase and haemagglutinin antibodies are important in providing protection against the influenza virus. We have, therefore, developed an automated neuraminidase assay, based on the enzymic method of Kendal, but modified to minimise the interference by sucrose. A neuraminidase standard has been established and samples assayed against the standard have a coefficient of variation of plus or minus 6.8 percent. The assay has been adapted to measure the neuraminidase antibody level in serum. The neuraminidase to haemagglutinin ratio has been determined for various influenza strains. Both neuraminidase and haemagglutinin titres are being compared with results obtained by the single radial diffusion method.