In February 1980, the World Health Organization convened a meeting to consider information relevant to the nomenclature of influenza viruses and to make definitive proposals for the revision of the system which has been in use since 1971. The WHO recommendations are based on data derived from double immunodiffusion reactions involving haemagglutinin and neuraminidase antigens. The revised system of nomenclature is similar to the 1971 system in that it consists of two parts: (a) a type and strain designation, and (b) for influenza A viruses, a description of the antigenic specificity (subtype) of the surface antigens (H and N). The strain designation for influenza virus types A, B, and C contains information on the antigenic type of the virus (based on the antigenic specificity of the nucleoprotein), the host of origin (for strains isolated from non-human sources), geographical origin, strain number, and year of isolation. For influenza A viruses, the antigenic description, in parentheses, follows the strain designation and comprises two indices describing the antigenic subtype of the haemagglutinin and of the neuraminidase antigens. For the influenza A viruses from all species, the H antigens are grouped into 12 subtypes, H1-H12, while the N antigens are divided into 9 subtypes, N1-N9. Reference strains of influenza viruses are maintained by the WHO Collaborating Centres for Reference and Research on Influenza and the WHO Centres for the Study of Influenza Ecology in Animals, and are made available upon request.There is no provision for describing distinct subtypes of influenza B and C viruses. The existence of antigenic variation among influenza B strains is well established but the available information shows that a division into subtypes is not warranted.This revised system of nomenclature should be used universally from the date of publication of this Memorandum.