[Comparative characteristics of epidemic strains of the influenza virus B isolated in 1980-1981]. 
The epidemic situation of 1980-1981 was conditioned by the emergence of new antigenic variants of influenza B virus with an altered composition of hemagglutinin and neuraminidase.  The strains isolated in the epidemic season of 1980-1981 vary in their antigenic and biological properties and the degree of relationship with previously circulating influenza B viruses, although they show sufficient stability of molecular weights of major virion polypeptides.  Influenza B viruses occurring after 1972 should be classified into a new, 5th group.  The epidemic strains which were spread in 1980-1981 belong to this group although they represent already new drift variants.