Objective Both the 2,6 linkage and its topology on target cells are critical for the recognition by human influenza virus.The binding preference of avian flu virus H5N1 HA to the 2,3-linked sialylated glycans is considered the major factor limiting its efficient infection and transmission in humans.To monitor potential adaptation of H5N1 virus in human population,the surveillance of receptor-binding specificity was undertaken in China.Methods The binding specificity of 32 human H5N1 virus strains isolated from 2003 to 2009 was tested by 2,3-specific sialidase-treated chicken red blood cell (CRBC) agglutination assay and a solid-phase direct binding assay with synthetic sialylglycopolymers.Results Dual binding preference to 2,3 and 2,6-glycans were found in two strains:A/Guangdong/1/06 (A/GD/1/06) and A/Guangxi/1/08 (A/GX/1/08).Though minor effect of short-2,6-binding was detected in A/GX/1/08 at a low virus titer,both showed high affinity to the oligosaccharide at a high load.Notably both are of the long-2,6-recognition,with the same topology as that of human H1N1 and H3N2 viruses.Conclusion The findings suggest that human H5N1 virus in China likely acquired the potential human-adaptation ability.Further research and surveillance on receptor-binding specificity of H5N1 viruses are required.