Confirmed reports of large domesticated cats becoming infected with highly pathogenic avian influenza (HPAI) H5N1 virus have raised questions about both the risk of infection for these animals, and their potential as vector or reservoir hosts in an influenza pandemic. With this in mind, we examined the immunogenicity of the hemagglutinin (HA) of H5N1 strain A/Vietnam/1203/04 using several different vaccination strategies. Data from ELISA assays showed that vaccination with a single dose of recombinant H5 HA protein induces a robust antibody response against both whole inactivated virus and recombinant HA antigen. Moreover, a single dose of the recombinant H5 HA protein induced hemagglutination inhibition titers >or=40, which is indicative of protective immunization. Cats receiving the IND H5N1 vaccine required two doses before similar H5 HA-specific antibody titers were observed, and despite boosting, these animals had HIA titers that were lower than or equivalent to those in the group receiving one injection of recombinant protein. In contrast, cats vaccinated with plasmid DNA encoding HA failed to develop HA-specific antibody responses above those seen in cohorts receiving an unrelated control plasmid. The results of this study indicate that recombinant H5 HA protein-based vaccines can rapidly induce high serum antibody titers, and may be more effective than either inactivated influenza virus or DNA vaccines in cats.