Marine, W. M. and W. M. Workman (Emory Univ. School of Medicine, Atlanta, Ga.). Hong Kong influenza immunologic recapitulation. Amer. J. Epid., 1969, 90: 406–415.—Antibodies to the 1968 Hong Kong variant strain of influenza A were found in sera obtained in 1964 from persons in an Atlanta, Georgia, nursing home. The prevalence was sharply related to age—85% in 118 persons bom 1869–1891 (pre-1892) and 15% in 27 persons born 1892–1901. The prevalence of antibodies to the Asian prototype, Japan/305, was equal in the two age groups and significantly lower than that to the Hong Kong variant in the pre-1892 group. The pre-1892 group showed minimal boosting of Hong Kong variant antibodies in 1964 after Asian vaccine; immunization with Hong Kong variant vaccine in 1968 showed no rise in Asian titer, emphasizing the difference between the Asian and Hong Kong variant strains. A relationship between the Hong Kong variant and an equine influenza, A/Eq 2, was established by: a) similar temporal distribution though lower prevalence of A/Eq 2 antibodies, and b) rise in A/Eq 2 antibodies in the pre-1892 group following Hong Kong variant vaccine. The hypothesis is made that previous circulation of a Hong Kong-like virus is responsible for Hong Kong variant and A/Eq 2 antibodies in pre-epidemic sera from the elderly. It is further proposed that the 1889–1890 pandemic virus was most closely related to the Hong Kong variant of all presently known strains.