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Summary Fourteen of 30 horses with pre-existing hemagglutination inhibition (HI) antibodies to A/Equine-1/Prague/1/56 virus exhibited 4-fold or greater rise in titer after infection with the A/Equine-2/63 agent. These findings give further evidence of shared viral antigens among the two families of Type A/Equine virus. HI and neutralizing antibodies to A/Equine-2/63 virus were found in human serum from individuals born between 1870 and 1900. Persons born about 1880 to 1890, however, exhibited higher levels and higher frequencies of antibody than did younger or older individuals. The results suggest a relationship between hemagglutination (HA) antigens of the 1963 equine strains and a virus formerly prevalent in man.