Background: Human antibodies that interfere with the biological activity of haemagglutinins (HAs) of influenza viruses have high potential as an antiviral agent. Methods: Human single-chain antibody fragments (HuScFv) to recombinant and native HAs of the influenza virus H5N1 subtype were produced using a human antibody phage display library with the intention to increase the therapeutic arsenal against this highly pathogenic virus. Results: The HuScFv inhibited HA activity and neutralized infectivity of both homologous and heterologous strains and clades of the H5N1 subtype in Madin-Darby canine kidney cell cultures. Intraperitoneally injected HuScFv also mediated immunotherapeutic protection in mice that were intranasally challenged with highly pathogenic H5N1 viruses belonging to different strains and clades. Conclusions: Our data indicate that it might be worth pursuing these HuScFv further for future consideration as candidates for influenza intervention and treatment. © 2009 International Medical Press.