The serology of influenza viruses typically uses hemagglutination inhibition (HI) or the neutralization test (NT). However, the sera of many humans and animals contain nonspecific inhibitors of hemagglutinin that must be inactivated or removed from the serum before use in the HI assay. Any nonspecific inhibitor in human serum is typically inactivated by pre-treatment with receptor-destroying enzyme (RDE). However, during the 2006/07 influenza circulating season, we observed that influenza vaccine strain A/Hiroshima/52/ 2005 (H3N2) exhibited susceptibility to an RDE-resistant inhibitor in human serum. We report herein on a preliminary characterization of this inhibitor, including the development of a novel inhibitor-inactivating technique for pre-treatment of human serum to be used for HI with the A/Hiroshima/52/2005 (H3N2) virus.