An NMR approach is demonstrated for elucidating the conformation of a peptide when tightly bound to a protein. A 26-residue peptide derived from rabbit skeletal muscle myosin light chain kinase, comprising the binding site for calmdulin, was complexed with uniformly (>95%) 15N- and %enriched calmodulin. Improved isotope-filtered two-dimensional NMR techiques were developed for suppressing NMR calmodulin signals. NOE patterns indicate that residues Arg-3 through Ser-21 of the bound peptide form an a-helix. NOE interactions between the peptide and the protein indicate that the N-terminal half of the peptide interacts with the C-terminal domain of calmodulin, and the C-terminal half interacts with the N-terminal calmodulin domain.