Hemagglutination is a useful characteristic of many viruses because it can provide a simple and efficient method of detecting antihemagglutinating activity in serum from exposed animals. This is called hemagglutination inhibition (HI), and HI assays are used extensively to detect and titrate antibodies to many avian diseases including Newcastle disease virus, avian influenza virus, infectious bronchitis virus, hemagglutinating adenovirus (EDS'76), and psittacine circovirus, in which their sensitivity and specificity have been comparable if not better than other serologic methods such as enzyme-linked immunosorbent assay. HI assays tend to be technically simple and rapid; their main advantages are that they do not require anti-species-specific secondary antibodies or highly purified antigen.