Antibody to the neuraminidase antigen of influenza A/Hong Kong (H3N2) virus was induced in adult volunteers by parenteral immunization with inactivated X-32 (HeqlN2) vaccine. Subsequently, vaccinated volunteers and a control group of men were challenged intranasally with live A/Hong Kong virus (H3N2). While the frequencies of infection in control and vaccinated groups were not significantly different, the incidence of illness in volunteers who received X-32 vaccine was significantly reduced. Rechallenge of available volunteers demonstrated complete protection of men vaccinated with X-32 who had been infected during the first challenge. These observations indicate that administration of. a neuraminidase-specific inactivated virus vaccine can induce protection against illness caused by infection with influenza virus containing an antigenically identical neuraminidase antigen and that the infection that occurs induces resistance to subsequent challenge with antigenically similar virus.