Four influenza A strains isolated from turkeys in Ontario as well as strain Chicken/Scotland/59 were found to immunize turkeys against the lethal disease caused by virus Turkey/Ontario 7732/66 (V7732). These viruses form the avian influenza A serotype 5 (AA5). Immunoprotection against V7732 was not obtained with influenza viruses of other serotypes. Immunoprotective relationships among AA5 viruses were not always demonstrable in hemagglutination-inhibition (HI) and serum-neutralization (SN) tests, especially with turkey and mammalian antisera; better correlation between immunoprotection and these in vitro tests was seen with chicken antisera. The implications of these findings relative to virus classification, serodiagnosis and vaccination are discussed.