Avian influenza A viruses belonging to hemagglutinin (HA) subtypes H5 and H6 were studied in the infectivity neutralization test and radioimmunoprecipitation assay (RIPA) with monoclonal antibody MAb C179. This MAb recognizes a conformational antigenic epitope in the stem region of HA formed by two regions (amino acid positions 318-322 in HA1 subunit and 47-58 in HA2), conserved in all H1 and H2 influenza viruses. MAb C179 reacts with HA of H5 viruses in RIPA and neutralizes these strains as efficiently as H2 viruses. C179 precipitates H6 subtype HA but does not neutralize the infectivity of these viruses. Comparison of amino acid sequences of H2, H5, and H6 strains showed identical epitope recognized by MAb C179 in H5 and H6 HAs, which differs from epitopes of H1 and H2 by two amino acids in the HA2 subunit. Causes of disagreement between immunoprecipitation of H6 HA by MAb C179 and neutralization of this serosubtype by this MAb are discussed.