Serological studies in the Netherlands have shown that exposure to A2 influenza viruses during the period 1957-67 increased the level of haemagglutination-inhibiting (HI) antibody against A/Equi-2 virus in the sera of persons aged 60 years or more and against A2/Hong Kong/68 virus in sera from persons less than 60 years old. On the other hand, HI antibody titres against A/Swine/30, A/PR/8/34 and A1/Nederland/49 viruses were not reinforced by exposure to A2 viruses.From these and other findings, the author concludes that the influenza virus strains of the A/Swine era are distantly related antigenically to those of the A2 era, that the pandemic virus of 1889 most closely resembles that of the 1957 pandemic, and that the Hong Kong-type virus appeared in man about 1900 and is probably responsible for A/Equi-2 antibodies found in human sera.