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SUMMARY Ten strains of avian influenza A viruses, naturally pathogenic for birds and grown in chick embryo allantois were compared by haemagglutination-inhibition and neuraminidase-inhibition tests, using chicken antisera. The viruses segregated into different cross-reaction groups in the two tests, emphasizing the need to include an assessment of neuraminidase antigens in the taxonomy of influenza A viruses. Four major groups of viruses were observed in the haemagglutination-inhibition test but several very low titre cross-reactions were constantly found outside these groups. Some of these reactions, not all of which were two-way, corresponded to anti-neuraminidase cross-reactions. The results of neuraminidase-inhibition tests, using a largely automated technique and a human serum fraction as substrate, confirmed our previous results obtained with a smaller substrate. The virus neu- raminidases could be separated into five separate antigen groups, without cross- reactions between the groups.

Studies with Avian Influenza A Viruses: Serological Relations of the Haemagglutinin and Neuraminidase Antigens of Ten Virus Isolates