Sera from people born between 1883 and 1930 and collected in 1977 were tested for the presence of HI antibodies to A/FM/1/47 (H1N1) virus and three recently (1977 and 1978) isolated influenza A-H1N1 viruses. The highest frequency of high-titred antibody to the four H1N1 viruses was detected in sera from people born in 1903-4, i.e. 42, 54, 38, and 22% had antibody against A/FM/1/47, A/Hong Kong/117/77, A/Brazil/11/78, and A/Fukushima/103/78 respectively. The birthdate groups 1896-1907 showed a higher percentage of HI antibody titres greater than or equal to 18, greater than or equal to 50, greater than or equal to 100 or greater than or equal to 1600 against the four H1N1 viruses than the birthdate groups 1907-30. This indicates the existence of an era, 1908-18, in which, apart from the H3N2 virus (1900-18), the H1N1 virus was epidemic among the human population.