U nderstanding the mechanisms underlying the poor immunogenicity of human self/tumor antigens is challenging because of experimental limitations in humans. Here, we developed a human-mouse chimeric model that allows us to investigate the roles of the frequency and self-reactivity of antigen-specific T cells in determination of the immunogenicity of an epitope (amino acids 209–217) derived from a human melanoma antigen, gp100. In these transgenic mice, CD8⁺ T cells express the variable regions of a human T cell receptor (hTCR) specific for an HLA-A*0201–restricted gp100_209–217. Immunization of hTCR-transgenic mice with gp100_209–217 peptide elicited minimal T cell responses, even in mice in which the epitope was knocked out. Conversely, a modified epitope, gp100_{209–217(2M)}, was significantly more immunogenic. Both biological and physical assays revealed a fast rate of dissociation of the native peptide from the HLA-A*0201 molecule and a considerably slower rate of dissociation of the modified peptide. In vivo, the time allowed for dissociation of peptide-MHC complexes on APCs prior to their exposure to T cells significantly affected the induction of immune responses. These findings indicate that the poor immunogenicity of some self/tumor antigens is due to the instability of the peptide-MHC complex rather than to the continual deletion or tolerization of self-reactive T cells.