The amide proton magnetic resonances of the λ-cro repressor protein have been examined. In order to assign them to specific amino acid residues, we labeled the cro protein by incorporating 15N-enriched leucine or 15N-enriched lysine and then conducted 1H/15N heteronuclear multiquantum coherence (HMQC) spectroscopy of the labeled cro protein. The chemical shifts of the amide proton resonances were titrated by the addition of operator DNA fragments. From the behavior of the shifts on binding to the operator DNAs, the DNA binding surface of the cro protein has been deduced. On the basis of the results with DNAs with different base sequences, the origins of the specific and nonspecific interactions are discussed.