Through extensive isolation of neutralizing mAbs against H3N2 influenza viruses representing the in vivo repertoire in a human donor, we examined the relationships between antigenic drift of influenza virus and protective antibodies generated in an infected individual. The majority of mAbs isolated from a donor born in 1960 were divided into three major groups with distinct strain specificity: 1968-1973, 1977-1993 and 1997-2003. In the present study, we developed a new method that allowed us to comprehensively determine the location of epitopes recognized by many mAbs. Original haemagglutinins (HAs) of several strains and chimaeric variants, in which one of the seven sites (A, B1, B2, C1, C2, D or E) was replaced by some other strain-derived sequence, were artificially expressed on the cell surface. The binding activity of mAbs to the HAs was examined by flow cytometry. By using this method, we determined the location of epitopes recognized by 98 different mAbs. Clones that neutralize the 1968-1973 strains bind to site B2/D, A or A/B1. While sites C, E and B were recognized by clones that neutralized the 1977-1993 strains, the majority of these clones bind to site C. Clones that neutralize the 1997-2003 strains bind to site B, A/B1, A/B2 or E/C2.