Eight convalescent human sera obtained from patients aged 3 to 14 years old, who were infected with influenza A(H3N2) virus during the 1990/1991 influenza season, were characterized by a binding assay with chimeric hemagglutinin (HA) proteins between influenza virus A/Aichi/2/68 and A/Kamata/14/91(H3N2) strains. These sera did not recognize the HA protein of the A/Aichi/2/68 strain but recognized that of the A/Kamata/14/91 strain. The binding assay revealed that these sera recognized only the HA1 domain of A/Kamata/14/91 HA protein. A further assay of the binding of these sera to the chimeric proteins of the HA1 domain revealed that three sera (A-1, A-2, and A-3) from very young patients bound only to region 150–170 (site B1) and one serum (Y-1) bound to regions 96–150 (site A) and 96–170 (sites A and B1). These four sera showed reduced hemagglutination inhibition (HI) activity with the 203v2 strain, a monoclonal variant of the A/Kamata/14/91 strain with two amino acid changes in the HA protein at antigenic sites A and B1. The other four sera (Y-2, G-1, G-2, and A-4) bound to regions 1–96 (site C/E), 96–150 (site A), 96–170 (sites A and B1), and 170–200 (site B2), two of which further bound to region 240–306 (site C); these sera were all fully reactive with the 203v2 strain. All eight sera showed reduced HI reactivity to a drift strain A/Aichi/4/93. Amino acid changes of the A/Aichi/4/93 strain from the A/Kamata/14/91 strain were located at antigenic sites A, B1, B2, and C. We propose a possible model for the emergence of a drift strain A/Aichi/4/93 from an A/Kamata/14/91-like strain by sequential changes during reinfections of individuals starting from A-1-like, next to Y-1-like, and then to Y-2-like populations.