Two split influenza virus vaccines administered intragastrically induced lower titres of haemagglutinin (HA)-specific antibodies in pulmonary secretions than whole virus vaccine or a third split virus vaccine. IgA antibody was the predominant HA-specific Ig class. HA-specific IgA titres decayed substantially within 2 weeks following booster immunization, but persisted for at least another 3.5 months. In contrast, HA-specific IgG was maintained at low titres throughout the 4 month study period. When the total vaccine antigenic mass was administered as one dose or as equally divided doses spread over several days, pulmonary antibody responses were comparable. Mice immunized intragastrically with whole virus vaccine were completely protected against intranasal challenge with a homologous virulent virus of the H3 subtype. Partial protection was obtained when the vaccine used for immunization was a distantly related, antigenically variant strain of the same subtype, but no protection was obtained with a monovalent vaccine of an influenza A subtype different to the challenge virus. These characteristics of the response to intragastric immunization against influenza are consistent with features of a useful vaccine.