AbstractThe interaction of co‐stimulatory molecules CD80/CD86 on antigen‐presenting cells with CD28 on naive CD8+ cytotoxic T (Tc) cells is understood to be critical in the induction of Tc effectors. CD80 is capable of providing signal 2 for the activation of Tc cells, but has no effect if encountered in the absence of specific peptide/MHC complexes (signal 1). We have found that CD80 presented in vitro to resting memory viral‐immune or alloimmune Tc cells can provide sufficient stimulus for the generation of effector Tc cells in the absence of specific antigen, the peptide/MHC class I complex. Effector Tc cells generated in vitro from influenza‐ or class I alloantigen‐primed mice by co‐stimulation in the absence of antigen require exogenous interleukin (IL)‐2 signaling via the cell surface‐expressed IL‐2 receptor or, under conditions of IL‐2 blockade, exogenous IL‐7. Activation of memory Tc cells by signal 1 and 2 is independent of IL‐2 and IL‐7. Although memory influenza‐immune Tc cells did respond to CD80 in the absence of antigen, the presence of antigen +CD80 enabled an earlier induction of these Tc cells and they retained their lytic activity in vitro over a longer time period. The capacity of memory Tc cells to be activated by signal 2 alone provides one explanation for the observed heterogeneity of phenotype of memory T cells in vivo and a possible mechanism for the maintenence of memory in the absence of persisting antigen.