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The results of virological and serological study of the influenza epidemic of 1976 revealed a further antigenic drift of hemagglutinin and neuraminidase in current influenza A viruses. One of the features of the A/76 epidemic strains was a marked weakening of antigenic relationships with viruses of the A/Port Chalmers group and a marked relation to chronologically more remote ancestor of the current epidemic subcycle, A/Victoria/72 virus. This gives grounds to consider the new viruses to be the progeny of the latter and not of A/Port Chalmers/73 viruses. The strains of influenza A virus isolated 1 1/2-2 months before the outbreak (November-December, 1975) were typical members of the A/Port Chalmers group and differed markedly from the agents in the epidemic of 1976. This evidence indicates the exogenic, imported origin of the causative agents of the influenza epidemic in 1976 in the USSR.

Investigations of the antigenic and biological properties of the causative agents of the 1977 influenza epidemic revealed their appurtenance to the old subtype A1 of influenza virus which had disappeared from circulation 20 years ago. A comparative study of A1 viruses isolated in 1947--1956 and A1/77 established closer relationship of the latter to the causative agents of 1949 and 1950 epidemics. The "old" and newly isolated viruses 4ere also closely related biologically. The similarity of influenza A1/77 viruses with the agents of the pandemic cycle of A1 in 1947--1956 is a convincing confirmation of the hypothesis of Francis-Davenport and Smorodintsev-Luzyanina.