We studied the size and avidity of primary and recall CD8+ T cell responses in vivo in mice immunized with dendritic cells presenting different densities of a MHC class I-restricted peptide. Increasing the epitope density on a fixed number of dendritic cells increased the size of the primary response, yet had no influence on the avidity of the effector cells. However, epitope density-based selection of cells with different avidities was evident in the subsequent memory population, and in recall responses. Additionally, mice primed with different peptide densities had similarly sized quiescent memory and recall responses. Our findings provide evidence for an important role for epitope density in the selection of T cells in vivo.