MAb 9F4 provides heterologous protection against multiple influenza A H5N1 clade viruses, including one of the recently designated subclades, namely 2.3.4, through binding to a novel epitope. The present invention relates to isolated mouse-human chimeric (xi) IgG1-9F4 and IgA1-9F4 MAb which retain high degrees of binding and neutralizing activity against influenza H5N1. The invention also relates to methods of production, kits and uses of the chimeric antibodies in the treatment of influenza A subtype H5N1 disease.