[Biological properties of the agents of the influenza epidemic of 1972-1973 in Leningrad and other cities of the Soviet Union].

The novelty of influenza epidemics of 1972-1973 in Leningrad and other towns of the Soviet Union was due to changes in the antigenic profile of influenza viruses neuraminidase which was found to be closely related to that of A/Victoria/35/72 virus according to the results of neuraminidase activity inhbition test. The epidemic influenza A virus strains isolated in 1972-1973 were also found to be sensitive to thermostable inhibitors of mouse serum which was not typical for the Hong Kong group of viruses.