Concanavalin A (Con A)-activated cytotoxic lymphocytes have been investigated, mapping the genetic differences between the P815 target and the effector cells required for cell-mediated lympholysis to occur. The target antigens recognized during the effector phase and the phenotype of the killer cell population(s) were also determined. It was found that Con A could activate a population of primed cytotoxic lymphocytes capable of killing target cells that were identical at the major histocompatibility complex (MHC) but differed at other background genes. Thus, after in vivo priming with DBA/2, B10.D2 lymphocytes cultured with Con A were capable of killing the P815 target. Unprimed B10.D2 cells, however, would not. Studies on the involvement of the MHC indicated that differences in the H-2K through H-2S, as well as differences in H-2D and H-2L alone could cause lysis. This killing could not be accounted for by additional differences at Qa-2, a MHC-linked locus. However, the contribution of other similar non-MHC linked loci could not be excluded. Cold target competition experiments indicated that MHC encoded alloantigens were involved as recognition structures on the target cell surface. Antisera plus complement depletion of cytotoxic effector function demonstrated that the cytotoxic cells had the cell surface phenotypes Thy 1.2+, Lyt 2.2+ and natural killer (NK) 1.1-. We conclude that Con A polyclonally activates population(s) of T cells that express antigen-specific cytotoxicity through clonally distributed recognition receptors intrinsic to their membranes when lectin is omitted from the cytotoxic assay.