We recently described a novel way to isolate populations of antigen-reactive CD4⁺ T cells with a wide range of reactivity to a specific antigen, using immunization with a fixed dose of nominal antigen and FACS^® sorting by CD4^high expression. Phenotypic, FACS^®, functional, antibody inhibition, and major histocompatibility complex–peptide tetramer analyses, as well as T cell receptor Vβ sequence analyses, of the antigen-specific CD4^high T cell populations demonstrated that a diverse sperm whale myoglobin 110–121–reactive CD4⁺ T cell repertoire was activated at the beginning (day 3 after immunization) of the immune response. Within 6 d of immunization, lower affinity clones were lost from the responding population, leaving an expanded population of oligoclonal, intermediate affinity (and residual high affinity) T cells. This T cell subset persisted for at least 4 wk after immunization and dominated the secondary immune response. These data provide evidence that CD4⁺ T cell repertoire selection occurs early in the immune response in vivo and suggest that persistence and expansion of a population of oligoclonal, intermediate affinity T cells is involved in CD4⁺ T cell memory.