Quantitative hemagglutination inhibition studies of the pre‐ and postimmune sera of nine human volunteers immunized with hog A substance have confirmed earlier findings that the character of the agglutinins is altered by immunization. Concomitant with immune antihog A antibody production, the slope of the probit‐log hemagglutination inhibition assay curve became steeper, the amount of hog A substance required to inhibit the agglutinins became smaller, and the rate of dissociation of hog‐A‐anti‐A complexes became slower. The observed differences in the reactivativities of these pre‐ and postimmune sera could be attributed to an increase in specificity and firmer binding of the antibodies to the inhibito