Complexes formed between a monoclonal anti-peptide Fab' and its complementary 15N-labeled peptide were studied primarily by isotope-edited nuclear magnetic resonance (NMR) techniques. The monoclonal antibodies used were raised against peptides corresponding to residues 69-87 of the protein myohemerythrin. The complexes studied correspond to Fab' bound to the synthetic peptide, MHKDFLEKIGGL-NH2 (residues 76-87 of myohemerythrin) labeled with 15N at various amides. The combined approach of using specifically 15N-labeled peptides and reverse-detection solution NMR techniques has allowed us to monitor selectively the peptide component of the Fab'-peptide complexes studied. Through the use of these techniques, we have been able to directly observe the resonances of the bound peptide, as well as distinguish between free and bound peptide resonances in situations when excess peptide is present. NMR titrations of the Fab' with antigen have shown that there is one site to which the peptide strongly binds. The NMR parameters of the various residues examined thus far have been quite distinctive from each other, reflecting the ability of these techniques to detect differences in the local environment of the individual residues. Differences with respect to these parameters among the various residues may be related to the overall antigenic properties of the peptide.