Accurate and timely diagnoses are central to H5N1 infection control. Here we describe the cloning and expression of the HA1 protein of the A/Vietnam/1203/04 strain in a bacterial system to generate mono-/polyclonal antibodies. All of the eight generated monoclonal antibodies recognized the same linear epitope on the top globular region of the HA structure-a highly conserved epitope among all circulating H5N1 clades identified by amino acid alignment. Results from fluorescence staining and Western blotting indicate that all monoclonal antibodies interacted with a denatured form of HA proteins, while the, resultant polyclonal antibodies recognized both denatured and native HA proteins on H5N1 reverse-genetics (RG) viruses. Results from flow cytometry and microneutralization assays indicate that the polyclonal antibodies blocked viral binding and neutralized H5N1-RG viruses. Our results may prove useful to establishing future H5N1 mono-and polyclonal antibodies, and perhaps contribute to the development of an alternative H5N1 vaccine. (C) 2009 Elsevier Inc. All rights reserved.