Hemagglutinin molecule is an envelope protein of influenza virus and plays an important role in the infection to human cells. Many mutations are observed in the molecule, which generates sixteen subtypes (H1-H16) of the hemagglutinin molecule for influenza virus A type. The subtypes such as H1, H2, H3, and H5 out of the sixteen are underlined molecules, which are responsible to Spain, Asia, Hong Kong, and Avian Flu, respectively. Based on the sequence analysis, three short sequences, which are highly conserved in the subtypes of influenza virus A type, were extracted. The sequence peptides were chemically synthesized and conjugated with BSA for immunization into Balb/c mice. A sequence GMVDGWYG located at the domain of fusion protein in the hemagglutinin molecule exhibited a high immuno-response, resulting in the production of a monoclonal antibody (mAb; InfA-15). The unique features of InfA-15 mAb were investigated from the viewpoint of immunological reaction, the binding affinity, the steric conformation, etc. The InfA-15 mAb could react with the H1, H3, and H5 subtype of hemagglutinin molecule of influenza virus A type. ELISAs using InfA-15 mAb suggested a wide reaction spectrum for the hemagglutinin of many important influenza viruses A type. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.