HA is a homotrimeric transmembrane protein with a globular head and a stem region that are both exposed to the outside of the membrane. These regions contain N-linked oligosaccharides, which have been known to affect the functional properties of HA. Glycosylation sites in the peptide sequences are highly conserved, indicating functional significance for HA glycosylation. HA is synthesized as a precursor protein that undergoes proteolytic cleavage into HA1 and HA2 subunits; HA1 1,7-Dimethylxanthine mediates initial contact with the cell membrane, and HA2 is responsible for membrane fusion. HA is the primary target for antiviral agents such as infectivity-neutralizing antibodies and nucleic acid aptamers. The recombinant HA1 subunit, expressed and purified from bacteria, induces an immune response against the influenza virus in humans and is sufficient for screening antiviral RNA aptamers. The recombinant HA protein, which retains glycosylation, has been expressed and produced in a baculovirus/insect cell system, which exhibited enhanced HA inhibition and virus neutralization. Aptamers are nucleic acid ligands that bind to a specific target molecule with high affinity. They are usually obtained from an oligonucleotide library DAF-FM DA harboring random sequences by using the SELEX method. Compared with protein antibodies, aptamers have many advantages over protein antibodies, such as high affinity, rapid synthesis, low cost, low-temperature sensitivity, large-scale production, and ease of chemical modification. To date, aptamers have been used in a wide range of applications as research reagents, for medical diagnosis, and as biosensor or therapeutic tools against viruses and cancer.. Previously, our group selected an RNA aptamer against HA1 of subtype H5 AIV, which specifically binds to HA1 and inhibits hemagglutination of erythrocytes in vitro. The HA1-specific RNA aptamer HAS15-5 was screened using the recombinant HA1�CGST fusion protein that lacks glycosylation because of its expression in bacteria. However, RNA aptamers specific to glycosylated HA rather than unglycosylated HA would be preferable for blocking and inhibiting influenza virus entry into host cells. In the present study, glycosylated hemagglutinin subtype H5 AIV was expressed from a recombinant baculovirus. RNA aptamers appear to contain a number of stem and loop structures, in which conserved sequences mostly reside at the stemand- loop region in the RNA secondary structure.