The present system of classification of influenza viruses was developed by WHO Expert Committees meeting in 1953 and 1959, and was based on the ribonucleoprotein and haemagglutinin antigens. Since then, another antigen, neuraminidase, has been identified and it has been found that the haemagglutinin and the neuraminidase undergo independent antigenic variation. It is therefore necessary that the system of classification should describe all three antigens. Furthermore, it is now known that both the haemagglutinin and neuraminidase antigens of influenza A viruses of human origin may be similar to, or identical with, those of strains from non-human hosts, and it is necessary that this antigenic relationship should be indicated. For influenza A viruses, therefore, a new system retaining as much as possible of the old system but also including the new information has been developed. It is the intention of WHO that the new system should be brought into use on 1 January 1972.