The age distribution of antibodies to A2/Hong Kong/68 influenza virus was ascertained in sera collected before the pandemic of 1968 in order to determine whether Hong Kong-like viruses had previously prevailed in man, and, if so, at what period. The findings indicate that Hong Kong-like viruses were probably involved in outbreaks at or about the turn of the century. The data are interpreted to indicate that antigenic drifting among Asian influenza strains from the A2/Japan/305/57-like variant, believed to be responsible for the pandemic of 1889-90, to the Hong Kong-like variant of 1900, is a phenomenon that has been repeated in recent times. The findings constitute another example of antigenic recycling after a long period of absence, and support the view that the number of antigens of influenza A is finite.