This study compared ligand densities on antigen-presenting cells (APCs) needed for in vitro restimulation of in vivo primed T cells and for in vitro assessed T cell effector function. Spleen cells of lymphocytic choriomeningitis virus (LCMV)-primed mice were restimulated in vitro with graded amounts of virus-derived peptides using macrophages or a cloned dendritic cell line as APCs. To test for effector function of these cytotoxic T cells, the same APCs pulsed with graded amounts of the peptides were used as target cells in an in vitro 51Cr release assay. The same peptide concentration that rendered an APC restimulatory for primed cytotoxic T lymphocytes (CTLs) also rendered it susceptible for lysis by the same CTLs. In addition, the same peptide concentrations that made macrophages susceptible for CTL-mediated lysis induced proliferative responses in vitro of in vivo primed memory CTLs. Thus, restimulation of in vivo primed T cells--measured by either proliferation or cytotoxic effector function--or sensibilization of target cells for lysis requires similar ligand densities on APCs and is therefore, contrary to expectations, governed by similar overall avidity thresholds. These results have implications for CTL memory.