Memory CD8(+) T cells are critical for host defense upon reexposure to intracellular pathogens. We found that interleukin 10 (IL-10) derived from CD4(+) regulatory T cells (Treg cells) was necessary for the maturation of memory CD8(+) T cells following acute infection with lymphocytic choriomeningitis virus (LCMV). Treg cell-derived IL-10 was most important during the resolution phase, calming inflammation and the activation state of dendritic cells. Adoptive transfer of IL-10-sufficient Treg cells during the resolution phase 'restored' the maturation of memory CD8(+) T cells in IL-10-deficient mice. Our data indicate that Treg cell-derived IL-10 is needed to insulate CD8(+) T cells from inflammatory signals, and reveal that the resolution phase of infection is a critical period that influences the quality and function of developing memory CD8(+) T cells.