We have used H-2Db-restricted CTL clones specific for peptide 365 to 380 of the influenza nucleoprotein to seek evidence for interaction between the TCR and peptide Ag. Preincubation of these CTL with peptide 365 to 380 resulted in inhibition of target cell lysis. In addition, CTL lysed allogeneic targets in the presence of soluble peptide Ag. Investigation of the basis of these two phenomena revealed a requirement for expression of H-2Db molecules by the effector cells. Either preincubation with anti-Db mAb or the use of chimera-derived H-2d CTL specific for Db plus peptide ablated both peptide-dependent inhibition and lysis of allogeneic cells, suggesting these activities are a consequence of self-presentation of peptide Ag by CTL. Lysis of allogeneic cells appears to represent bystander lysis by CTL in response to recognition of peptide on other effector cells. Lysis inhibition is attributable to a highly potent form of cold target inhibition in which CTL serve as their own cold targets.