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.