Abstract: A novel peptide obtained from Haemophilus paragallinarum has been found useful for preventing avian infectious coryza. This polypeptide induces production of hemagglutination-inhibition antibody and prevents infection and onset of avian infectious coryza. The invention further provides a gene coding for the polypeptide, a recombinant vector for expression of this gene, a host transformed with this vector, a process for preparing the polypeptide in a host, a vaccine for avian infectious coryza comprising the polypeptide as an active ingredient, a monoclonal antibody obtained using the polypeptide as an immunogen, and a diagnostic agent and a therapeutic agent for avian infectious coryza using the peptide and the antibody.