Haemagglutination-inhibition tests (HI) are used to detect increases in influenza antibody in serum. However, they are relatively insensitive for the detection of human antibody responses to avian haemagglutinin, even in the presence of high titres of neutralising antibody after confirmed infection or vaccination. Human influenza viruses bind preferentially sialic acid containing N-acetylneuraminic acid α2,6-galactose (SAα2,6Gal) linkages while avian and equine viruses bind preferentially those containing N-acetylneuraminic acid α2,3-galactose (SAα2,3Gal) linkages. Increasing the proportion of SAα2,3Gal linkages on the erythrocytes used, by enzymatic modification or change of species, improves the ability of erythrocytes to bind to avian influenza strains and thereby improves the sensitivity of detection of antibody to avian and equine HA in a range of mammalian and human sera using HI tests. J. Med. Virol. 70:391–398, 2003. © 2003 Wiley-Liss, Inc.