Previous workers had demonstrated that some of the viruses that caused the influenza epizootics of 1956 and 1963 among horses are antigenically related to certain human influenza viruses. The present authors therefore studied the distribution of antibodies for the equine influenza viruses in human sera. They found that three strains of A/Equi 2 virus (1963 Miami, 1963 Richelieu and 1963 Milford) are antigenically related and that there is a cross-relationship between the mouse line of the equine influenza A/Equi 1 (1956 Praha) antiserum and the 1963 A/Equi 2 strains. Haemagglutination-inhibition (HI) antibodies for A/Equi 2/Richelieu/63 were found in only 2 of 434 sera taken in 1963 from people aged less than 60 years but in 50 of 435 sera from people aged 60 years or more. These results suggest that infection with a virus resembling the A/Equi 2 1963 strain occurred some 7-10 years after the 1889-90 influenza pandemic; this virus was probably a mutant of the influenza A2 virus, which may have been the cause of the 1889-90 pandemic.