Over 950 randomly selected sera from various regional bacteriological laboratories, collected in October 1979, were assayed for the presence of haemagglutination-inhibition antibodies to current influenza strains. It was shown that in contrast to the previous year (1978), most age groups had significant levels of antibodies to the A/USSR influenza variant. In October 1978 only children, adolescents, and adults born between 1931 and 1950 had such antibodies. As there was little laboratory evidence of A/USSR infections in other age groups than among children and adolescents, we conclude that the majority of these infections must have been inapparent or generally mild.