The nuclear magnetic resonance titration curves of chemical shift versus pH of two of the histidine ring C(2) protons of ribonuclease show deviations from the curve expected for a simple proton association equilibrium. We have previously shown (Sachs, Schechter & Cohen, 1971), using model systems, that titrating groups in spatial proximity to an imidazole ring give rise to such effects and that these data can be analyzed by computer curve-fitting to give microscopic and macroscopic apparent ionization constants of the imidazole ring and the interacting groups. When applied to the curve with the greater deviation, such an analysis shows that the histidine residue giving rise to this resonance has a macroscopic apparent pK of 6-1 and is interacting with an adjacent group of pK 4-6. This resonance has previously been assigned to histidine residue 119 and, on the basis of information from X-ray crystallographic studies, we believe its interacting group is the carboxyl function of aspartic acid residue 121. The curve probably corresponding to histidine residue 12 has a macroscopic apparent pK of about 6-2 but displays small acid and alkaline inflections. These result from interactions with groups of pK 5 and 8.4. The alkaline group is possibly the ε-amino function of lysine residue 41, on the basis of the known three-dimensional structure. This method of analysis may be a general one for measuring interactions of charged groups with histidine residues in proteins.