AbstractThe plaque inhibition method of measuring the affinity of single antibody‐forming cells (AFC) developed by Andersson has been modified for use with haptens. Using this method, it was possible to make simultaneous determinations of the relative affinities of IgG and IgM plaques during the primary and secondary responses.It was shown that there was no maturation of the affinity of antibody‐released IgM AFC in the primary response in mice. There was an increase in affinity of the antibody released from IgG AFC, which was antigen dose‐ and time‐dependent, confirming results obtained by others. Furthermore, at any time during the course of the primary response, the affinity of the IgM antibody released by AFC was always lower than that of IgG plaque‐forming cells. This difference averaged about 10‐fold.The same factors were shown to influence the affinity changes of IgG AFC observed during the secondary response. In addition the booster, or secondary dose of antigen used, was important. Highest affinity antibody was produced in response to intermediate antigen doses. The maturation of affinity affects the generation of memory cells to the same degree as the antibody‐secreting cells during the primary response, since similar affinity changes occurred in the absence of detectable primary responses.Finally, when the period between primary stimulation and challenge is 5 months, there is a decrease of between 2 and 2.6 log10in the concentrations of DNP lysine needed to inhibit 50 % of the IgG antibody‐forming cells depending on the doses used for priming. The variation of affinity of IgM antibody was much less obvious, with a 6‐fold difference at the maximum, which was independent of the doses of antigen used for priming or boosting, and of the time elapsed between the two