A broad-spectrum viral antigen for the detection of avian-influenza-virus-specific antibodies, using the indirect enzyme-linked immunosorbent assay (ELISA), was identified. Purified and disrupted antigens were used, which helped to increase the sensitivity of the assay. All of the antigens tested were able to detect antibodies to homologous and heterologous viruses to varying degrees. The H9N2 antigen was the best single antigen to use in the ELISA to screen for avian influenza virus antibodies. It detected antibodies against six viruses as early as day 4 postinfection.