Summary Antiserum was prepared in rabbits against purified haemagglutinin obtained from A0/bel virus. The antiserum had potent virus-neutralizing and haemagglutination-inhibiting activity, but was free of antibody to A0/bel neuraminidase or to influenza A type-specific ribonucleoprotein. In immunodiffusion tests, the anti-haemagglutinin serum gave precipitin reactions with all A0 and A1 virus strains tested but did not react with human A2 viruses or avian or porcine influenza A viruses. The studies suggested a close immunological relationship between the haemagglutinins of A0 and A1 influenza viruses which was not revealed by haemagglutination-inhibition tests.