High-resolution proton magnetic resonance spectra between 11 and 14 ppm of bovine ribonuclease A in H2O show that a single resonance titrates with changes in pH. This resonance arises from a histidine ring nitrogen proton and exhibits an apparent pKa of 5.8. This pKa value can be correlated with that of only one of the four histidine ring C-2 protons, namely, that which has been assigned to the active site histidine-119. Additional evidence for the correlation is obtained from the selective shifts of the apparent pKa, values caused by temperature variation and by the binding of the inhibitors cytidine 2′-monophosphate, cytidine 3′-monophosphate, cytidine 5′-monophosphate, and inorganic phosphate. These results suggest that a specific histidine ring nitrogen proton of residue 119 is relatively inaccessible to solvent. Solvent inaccessibility of histidine residues as defined by proton magnetic resonance studies of proteins in H2O is discussed.