HLA‐A2.1‐binding peptides (n = 38) were screened for immunogenicity with human peripheral blood mononuclear cells in cytotoxic T lymphocyte (CTL) induction experiments in vitro and with splenocytes from HLA‐A2.1/K^b transgenic mice following immunization in vivo. These data were compiled and analyzed to determine the level of overlap between the A2.1‐restricted CTL repertoire of A2.1/K^b‐transgenic mice and A2.1⁺ humans. In both humans and mice, a major histocompatibility complex affinity threshold of approximately 500 nM appears to determine the capacity of a peptide to elicit a CTL response. Good concordance between the human data in vitro and mouse data in vivo was observed with 85% of the high‐binding peptides, 58% of the intermediate binders, and 83% of the low/negative binders. Although some peptides immunogenic for mouse CTL but not for humans (and vice versa) could be identified, the data as a whole suggest an extensive overlap between T cell receptor repertoires of mouse and human CTL and support the use of HLA‐transgenic mice for the identification of potential human CTL epitopes.