A comparison of the rates and dynamics of the maturation process in specific IgM and IgG immune responses was made at the cellular level in BALB/c mice immunized with an enlarged hapten, N-(2,4-dinitrophenyl)-β-alanylglycylglycine (DnpAGG) coupled to hemocyanin. At intervals after immunization, the relative binding characteristics of IgM and IgG plaque-forming cells (PFC) secreting anti-DnpAGG antibodies were determined by the plaque inhibition technique. With time after immunization, the concentration of free hapten required to cause 50% reduction in both IgM and IgG PFC numbers decreased indicating that an increase in average binding affinity of PFC antibodies occurred. This maturation process was complete for IgM PFC by 9 days after immunization but required about 30 days in the IgG PFC response. A subpopulation analysis showed that maturation of both IgM and IgG responses occurred as the result of a simple selective process involving initial stimulation of a heterogeneous population of DnpAGG-specific PFC followed by preferential retention of that subpopulation of PFC producing antibody of the highest affinity. The data indicated that the IgM response evolved from precursor cells present before specific immunization.