Individual residues derived from antigenic peptides are believed to control certain mAb epitopes on MHC class I molecules. To further evaluate this influence we studied the antibody recognition of H-2Kb complexed with the antigenic peptides OVA257-264, OVA55-62 and their reciprocally substituted analogs. The mAb 20.8.4, Y-3, EH144 and AF6 reacted strongly with Kb complexed to both OVA peptides and their analogs. However, mAb 28.8.6 bound to Kb/OVA257-264 but not Kb/OVA55-62 complexes. Recognition of Kb/OVA55-62 by mAb 28.8.6 was restored by a single OVA55-62-->OVA257-264 substitution at position 1 (OVA55-62P1K-->S). Recognition by mAb 5F1 was also peptide-dependent in that Kb complexed to either the P4 substituted OVA257-264-->OVA55-62 peptide (OVA257-264P4N-->R) or the P1 substituted OVA55-62-->OVA257-264 peptide (OVA55-62P1K-->S) was not permissive for 5F1 recognition even though Kb complexed to the parental peptides OVA55-62 (P4R) and OVA257-264 (P1S) reacted with this mAb. These data demonstrate that the same peptide residues located within defined positions can exert a negative influence on mAb recognition in one sequence context but be permissive for antibody binding in another context. These findings might be explained by conformational effects on class I heavy chain structure, the extended structure of the peptide or altered orientation of specific peptide side chains located at the same position but within different sequence contexts. Therefore the sequence context of defined amino acids within peptide antigen can strongly influence the fine structural contributions of these residues to MHC-peptide conformation.