Immunodominant T cell epitopes from the autoantigen human cartilage glycoprotein 39 have previously been mapped in the context of HLA‐DR*0401 and *0402, using mice expressing HLA‐DR4 transgenes. We measured the dissociation rates of these epitopes from soluble recombinant DR*0401 and DR*0402 to assess the relationship between peptide/HLA‐DR4 kinetic stability and immunogenicity. Experiments were performed at endosomal pH (5.5) and at cell surface pH (7), in the absence and presence of soluble recombinant HLA‐DM (sDM). All (4/4) immunodominant peptide/HLA‐DR complexes exhibit dissociation half‐times of 1 h to several days. In contrast, most (3/4) non‐immunodominant complexes dissociate with half‐times <30 min under at least one of these conditions. Interestingly, a complex which is stable except in the presence of HLA‐DM at pH 5.5 is immunogenic only following peptide immunization, while a complex which is stable at acidic but not at neutral pH, is non‐immunogenic following either whole protein or peptide immunization. These data indicate that kinetic stability of peptide/MHC complexes in vivo is a key determinant of immunogenicity.