Experimental studies in mice have provided evidence that antibody specific for viral neuraminidase markedly inhibits influenza virus replication in the lungs of animals challenged with virus containing homologous neuraminidase. In contrast to antihaemagglutinin antibody, antineuraminidase antibody does not increase resistance to the initiation of infection, but does reduce the capacity for subsequent transmission of infection.The 1968 Hong Kong virus was found to possess a haemagglutinin antigen markedly different from previous A2 strains but its neuraminidase is indistinguishable from the enzyme antigens of 1967-68 A2 viruses, and protection of mice immunized with 1957 and 1967 A2 viruse and challenged with Hong Kong virus was entirely attributable to antineuraminidase antibody.The desirability of systematically examining the protective effects of antineuraminidase antibody in man and of revising current methods for selection and standardization of influenza vaccines are discussed.