We observed that the volume of any anti-HSA serum needed to bind 50 per cent of the antigen is a linear function of the amount of antigen present in the reaction mixture over three log units. The slope is characteristic of each antiserum and reflects the `avidity' of the antibodies.
By using this quantitative index we found that the avidity of antibodies produced during the secondary adoptive response depends on the number of the cells transferred: few cells will produce antibodies of lower avidity than many cells. The dose of antigen given as a booster also influences the quality of the antibody produced; in the extreme case where some primed cells were rendered `tolerant' by supra-optimal doses of challenging antigen, substantial amounts of low avidity antibodies were synthesized. The results can be explained by competition for antigen among competent cells, favouring those cells which carry antibody sites with high affinity for the corresponding antigenic determinant.