Publisher Summary The chapter discusses the recent work on the acid-base dissociations of native proteins and unreactive prototropic groups in certain native proteins, with emphasis on conditions for their detection, and their relation to the protein denaturation and structural changes, which accompany it. The chapter discusses the theoretical analysis of titration curves and stoichiometry, both theoretical and practical, which are used to deduce from the titration data numbers and dissociation constants of prototropic groups, as well as other information bearing on the structure of individual proteins. The method used for the theoretical analysis of titration curves of soluble proteins accounts adequately, in most cases, for the shape of the curves in terms of the numbers and equilibrium constants of different kinds of dissociable groups and the electrostatic interaction of the protein with hydrogen ion and other ions. The prototropic groups of a number of native proteins, like their other functional groups, are less fully reactive than in the denatured protein, or than would be expected if all their side chains were freely available. Thus, in the hemoglobin of horse, pig, and human beings, large numbers of pairs of carboxyl and amino groups are hindered from dissociating or combining with hydrogen ion until after reaction of a smaller number of trigger groups initiates their liberation. The trigger groups in the heme proteins appear most likely to involve the propionate side chains of the prosthetic groups. The unmasking phenomenon is always accompanied by denaturation, as usually defined, although it is often fully reversible if the protein is not exposed for long periods to extremes of pH or temperature. More generally, denaturation (whether reversible or not) renders reactive to such prototropic groups as were not free to react in the native state.