Comparison of antibody content of pre- and postepidemic collections of sera pooled by ages showed an increase in the average antibody level of the population in the postepidemic period. Increase in antibody was most marked in the childhood age range when it was measured with recent A-prime isolates. This correlates with the known high incidence of influenza in childhood. Reinforcement of antibody to older strains, i.e., PR8 and swine virus, was demonstrated in adults. These results were in accordance with the concept that the initial infections of childhood orient the antibody-forming mechanism, so that experience later in life to antigenically similar strains confers progressive reinforcement of the primary antibody, which then characterizes that part of the population throughout life. The findings taken in conjunction with an analysis of antibody content of individual sera collected from consecutive age groups in a post-epidemic period demonstrated that with experience not only is the antibody mechanism oriented by the initial infections of childhood, but that, with experience, antibody reacting with successively prevalent strains is added. This results in a continuing broadening of the antibody spectrum with age which confers the immunity of the older age groups in the population. The analysis of the antibody levels of individual sera showed a few individuals in childhood and adolescence also to have antibody against major antigens of viruses which are no longer prevalent. This indicates that strains with major antigens of older viruses have had a limited circulation in recent years. The age at which antibody against older prototype strains appears and the age range at which antibody is present in greatest amount, as well as the general configuration of patterns of antibody distribution in the population, as shown in this study with pools of human sera and with groups of individual sera, are very similar results to those of the previous study (1). Three independent samples, then, demonstrated the serological recapitulation of the history of the experience with variants of influenza virus.