We performed radioimmunoprecipitation assays in which iodinated preparations of A/Port Chalmers/73 (A/PC/73) hemagglutinin were used as the test antigens and high concentrations of unlabeled A/Hong Kong/68 viral protein were used to inhibit the binding of cross-reactive antibodies to quantitate strain-specific antibody responses in postvaccination sera. Strain-specific antibodies comprised 8 to 48% (mean, 20%) of the total A/PC/73 antigen-binding capacity of the sera tested. Competition radioimmunoprecipitation assays in which disrupted preparations of purified whole virus representative of several of the H3N2 variants were used indicated that the A/PC/73 strain-specific antibody that was present after adsorption of serum by A/Hong Kong/68 antigen was capable of reacting with A/England/72 and A/Victoria/75 hemagglutinins, but generally with lower avidity than with A/PC/73 hemagglutinin. A comparison of the A/PC/73 antibody titers measured by radioimmunoprecipitation and hemagglutination inhibition tests before and after adsorption with A/Hong Kong/68 whole virus suggested that cross-reactive and strain-specific antibodies were comparable in efficiency of inhibiting viral hemagglutination. These data indicated that vaccines containing later variants within a subtype could induce antihemagglutinin antibodies of restricted specificity, but that these antibodies may not be directed against unique antigenic determinant(s).