Neuraminidase (NA) and hemagglutinin (HA) are the two predominant membrane glycoproteins found on the surface of an influenza virus particle. They are essential for the infectious cycle of the virus. HA recognizes and binds to the sialic acid on the host cell membrane to initiate a viral infection. NA cleaves the sialic acid at the end of the cycle, allowing the progeny virus to leave the host and initiate the next round of infection (1). In the early stage of an infection, NA may also assist in viral penetration of the mucus layer in the airway of a host. Nine subtypes of NA (N1 to N9) have been identified, all of which are believed to be tetrameric and share a basic structure consisting of a globular head, a thin stalk region, and a small hydrophobic region that anchors the protein in the virus membrane (2). Glycosylation has also found been important for the stability and activity of these enzymes (3, 4). According to a recent structure determination (5), there are two genetically distinct groups of neuraminidases from influenza type A viruses, with the N1 and N2 neuraminidases representing the two groups. Due to their critical role in the infectious cycle of a virus, influenza viral neuraminidases are frequently used as targets for drug design. In fact, both Tamiflu and Relenza, anti-influenza drugs, are neuraminidase inhibitors. Our recombinant H5N1 neuraminidase is based on the avian virus isolated from the 2004 outbreaks of the H5N1 virus in Vietnam and Thailand (6). H5N1 avian virus is one of the most lethal viruses in history (7, 8) with an accumulative death rate of 59% from 2003 to 2012 according to the World Health Organization (9). To produce active recombinant enzyme, a tetramerization domain from the vasodilator-stimulated phosphoprotein (10) was inserted at the N‑terminus to assist in oligomerization of the protein. We found that the activity of the recombinant H5N1 neuraminidase is activated by Ca ²⁺ and inactivated by Zn ²⁺, Cu ²⁺ and Fe ²⁺.