One set of experiments was conducted to understand the cross protection between paired H5N1 HA antigens. Rabbits were immunized with individual H5-HK.tPA, H5-VN.tPA, and H5-AH.tPA DNA vaccines and rabbit immune sera were examined for HA antigen-specific antibody responses. H5 HA-specific antibody responses against H5 HA antigens from different viruses were analyzed by ELISA and the potential cross-protective antibody responses against different H5N1 viruses were evaluated by HI and MN assays.