Immunoglobulin G directed against the hemagglutinin (HA) of influenza virus A/Memphis/102/72 (H3N2) was adsorbed with either of two related viruses, A/Port Chalmers/73 (H3N2) or A/Papua New Guinea/75 (H3N2) to fractionate the antibody into “strain-specific” (“specific”) and “cross-reactive” (“common”) populations. These IgG preparations were tested with use of a solidphase radioimmunoassay for their ability to compete with one another in binding to the antigenic determinants of Memphis HA. Their binding avidities were also determined. It was found that anti-“specific” and anti-“common” IgG preparations competed with each other for the antigenic regions on Memphis HA. In addition, the assignment of anti-HA antibodies to either the anti-“specific” or the anti-“common” category was shown to be completely dependent on the particular virus used for adsorption; the more distantly related the strains being compared, the higher the proportion of antibodies that registered as anti-“specific”, and the higher the average binding avidity of the anti-“specific” antibody population. It is concluded that so-called anti-“specific” anti-“common” antibodies are directed against the same regions on the HA molecule. This does not necessarily imply that there is only a single antigenic determinant; there may be more than one, each inducing the production of a heterogeneous population of antibody molecules binding with different avidities to that determinant.