To confirm that these antibodies can recognize the HA antigen, the reactivity of the anti-peptide sera were evaluated by Western blot and ELISA against the purified HA0 protein of H1N1pdm virus. Our data demonstrate that sera against P3, P5, and P31 but not those against P6 and P30 (controls) react to the HA0 protein ( Fig. 2B and 2C ). The anti-P16 sera did not react to the HA0 protein, although it exhibited a high ELISA reactivity to the HA0 protein ( Fig. 2B and 2C ). Taken together, our results demonstrate that P3, P5, and P31 peptides contain dominant epitopes of H1N1pdm virus. We then characterized these three peptides in the following studies.