It has long been an important issue to produce a catalytic antibody that possesses the ability to lose the infectivity of a bacteria or virus. The monoclonal antibody JN1-2 was generated using a synthetic peptide (TGLRNGITNKVNSVIEKAA) conjugated with human IgG. The peptide sequence includes the conserved region of the hemagglutinin molecule (HA(1) and HA(2) domains), which locates on the envelope of the influenza virus and plays an important role in influenza A virus infection. The monoclonal antibody specifically reacted with the HA2 domain, not only of H2 but also of an H1 strain of the H1N1 subtype (H1 strain). The heavy chain (JN1-2-H) isolated from the parent antibody showed catalytic activity cleaving the above antigenic peptide with very high turnover (kcat = 26 min(-1)), and it could slowly degrade the recombinant HA2 domain by the catalytic function. Interestingly, the heavy chain exhibited the ability to reduce the infectivity of type A H1N1 but not type B, indicating specificity to type A. This characteristic monoclonal catalytic antibody heavy chain could suppress the infection of the influenza virus in vitro assays.